Concurrent and cross-season protection of inactivated influenza vaccine against A(H1N1)pdm09 illness among young children: 2012-2013 case-control evaluation of influenza vaccine effectiveness

Maternal Immunizations: Past, Present, and Future

Maternal vaccines during pregnancy offer crucial protection against infections for both the pregnant person and their newborn. Vaccines against influenza, pertussis, coronavirus disease 2019, and respiratory syncytial virus are routinely recommended by the Centers for Disease Control and Prevention to safeguard pregnant women and their infants from potentially severe complications. Administering these vaccines during pregnancy helps transfer protective antibodies from the mother to the baby, enhancing immunity during the vulnerable early months of life. Extensive research supports the safety and efficacy of maternal vaccines, with numerous studies demonstrating their protective benefits for both pregnant people and newborns.

Does repeated influenza vaccination attenuate effectiveness? A systematic review and meta-analysis

BACKGROUND

Influenza vaccines require annual readministration; however, several reports have suggested that repeated vaccination might attenuate the vaccine's effectiveness. We aimed to estimate the reduction in vaccine effectiveness associated with repeated influenza vaccination.

METHODS

In this systematic review and meta-analysis, we searched MEDLINE, EMBASE, and CINAHL Complete databases for articles published from Jan 1, 2016, to June 13, 2022, and Web of Science for studies published from database inception to June 13, 2022. For studies published before Jan 1, 2016, we consulted published systematic reviews. Two reviewers (EJ-G and EJR) independently screened, extracted data using a data collection form, assessed studies' risk of bias using the Risk Of Bias In Non-Randomized Studies of Interventions (ROBINS-I) and evaluated the weight of evidence by Grading of Recommendations Assessment, Development, and Evaluation (GRADE). We included observational studies and randomised controlled trials that reported vaccine effectiveness against influenza A(H1N1)pdm09, influenza A(H3N2), or influenza B using four vaccination groups: current season; previous season; current and previous seasons; and neither season (reference). For each study, we calculated the absolute difference in vaccine effectiveness (ΔVE) for current season only and previous season only versus current and previous season vaccination to estimate attenuation associated with repeated vaccination. Pooled vaccine effectiveness and ∆VE were calculated by season, age group, and overall. This study is registered with PROSPERO, CRD42021260242.

FINDINGS

We identified 4979 publications, selected 681 for full review, and included 83 in the systematic review and 41 in meta-analyses. ΔVE for vaccination in both seasons compared with the current season was -9% (95% CI -16 to -1, I2=0%; low certainty) for influenza A(H1N1)pdm09, -18% (-26 to -11, I2=7%; low certainty) for influenza A(H3N2), and -7% (-14 to 0, I2=0%; low certainty) for influenza B, indicating lower protection with consecutive vaccination. However, for all types, A subtypes and B lineages, vaccination in both seasons afforded better protection than not being vaccinated.

INTERPRETATION

Our estimates suggest that, although vaccination in the previous year attenuates vaccine effectiveness, vaccination in two consecutive years provides better protection than does no vaccination. The estimated effects of vaccination in the previous year are concerning and warrant additional investigation, but are not consistent or severe enough to support an alternative vaccination regimen at this time.

FUNDING

WHO and the US National Institutes of Health.

Effects of Prior Season Vaccination on Current Season Vaccine Effectiveness in the United States Flu Vaccine Effectiveness Network, 2012-2013 Through 2017-2018

BACKGROUND

We compared effects of prior vaccination and added or lost protection from current season vaccination among those previously vaccinated.

METHODS

Our analysis included data from the US Flu Vaccine Effectiveness Network among participants ≥9 years old with acute respiratory illness from 2012-2013 through 2017-2018. Vaccine protection was estimated using multivariate logistic regression with an interaction term for effect of prior season vaccination on current season vaccine effectiveness. Models were adjusted for age, calendar time, high-risk status, site, and season for combined estimates. We estimated protection by combinations of current and prior vaccination compared to unvaccinated in both seasons or current vaccination among prior vaccinated.

RESULTS

A total of 31 819 participants were included. Vaccine protection against any influenza averaged 42% (95% confidence interval [CI], 38%-47%) among those vaccinated only the current season, 37% (95% CI, 33-40) among those vaccinated both seasons, and 26% (95% CI, 18%-32%) among those vaccinated only the prior season, compared with participants vaccinated neither season. Current season vaccination reduced the odds of any influenza among patients unvaccinated the prior season by 42% (95% CI, 37%-46%), including 57%, 27%, and 55% against A(H1N1), A(H3N2), and influenza B, respectively. Among participants vaccinated the prior season, current season vaccination further reduced the odds of any influenza by 15% (95% CI, 7%-23%), including 29% against A(H1N1) and 26% against B viruses, but not against A(H3N2).

CONCLUSIONS

Our findings support Advisory Committee on Immunization Practices recommendations for annual influenza vaccination. Benefits of current season vaccination varied among participants with and without prior season vaccination, by virus type/subtype and season.

Influenza Vaccine Effectiveness in Mainland China: A Systematic Review and Meta-Analysis

Influenza endangers human health but can be prevented in part by vaccination. Assessing influenza vaccine effectiveness (VE) provides scientific evidence for developing influenza vaccination policy. We conducted a systematic review and meta-analysis of studies that evaluated influenza VE in mainland China. We searched six relevant databases as of 30 August 2019 to identify studies and used Review Manager 5.3 software to analyze the included studies. The Newcastle–Ottawa scale was used to assess the risk of publication bias. We identified 1408 publications, and after removing duplicates and screening full texts, we included 21 studies in the analyses. Studies were conducted in Beijing, Guangzhou, Suzhou, and Zhejiang province from the 2010/11 influenza season through the 2017/18 influenza season. Overall influenza VE for laboratory confirmed influenza was 36% (95% CI: 25–46%). In the subgroup analysis, VE was 45% (95% CI: 18–64%) for children 6–35 months who received one dose of influenza vaccine, and 57% (95% CI: 50–64%) who received two doses. VE was 47% (95% CI: 39–54%) for children 6 months to 8 years, and 18% (95% CI: 0–33%) for adults ≥60 years. For inpatients, VE was 21% (95% CI: −11–44%). We conclude that influenza vaccines that were used in mainland China had a moderate effectiveness, with VE being higher among children than the elderly. Influenza VE should be continuously monitored in mainland China to provide evidence for policy making and improving uptake of the influenza vaccine.

Influenza vaccine effectiveness against hospitalizations in children and older adults-Data from South America, 2013-2017. A test negative design

BACKGROUND

In 2013, the Pan American Health Organization established a multi-site, multi-country network to evaluate influenza vaccine effectiveness (VE). We pooled data from five consecutive seasons in five countries to conduct an analysis of southern hemisphere VE against laboratory-confirmed influenza hospitalizations in young children and older adults.

METHODS

We used a test-negative design to estimate VE against laboratory-confirmed influenza in hospitalized young children (aged 6─24 months) and older adults (aged ≥60 years) in Argentina, Brazil, Chile, Colombia, and Paraguay. Following country-specific influenza surveillance protocol, hospitalized persons with severe acute respiratory infections (SARI) at 48 sentinel hospitals (March 2013-December 2017) were tested for influenza virus infection by rRT-PCR. VE was estimated for young children and older adults using logistic random effects models accounting for cluster (country), adjusting for sex, age (months for children, and age-in-year categories for adults), calendar year, country, preexisting conditions, month of illness onset and prior vaccination as an effect modifier for the analysis in adults.

RESULTS

We included 8426 SARI cases (2389 children and 6037 adults) in the VE analyses. Among young children, VE against SARI hospitalization associated with any influenza virus was 43% (95%CI: 33%, 51%) for children who received two doses, but was 20% (95%CI: -16%, 45%) and not statistically significant for those who received one dose in a given season. Among older adults, overall VE against SARI hospitalization associated with any influenza virus was 41% (95%CI: 28%, 52%), 45% (95%CI: 34%, 53%) against A(H3N2), 40% (95%CI: 18%, 56%) against A(H1N1)pdm09, and 20% (95%CI: -40%, 54%) against influenza B viruses.

CONCLUSIONS

Our results suggest that over the five-year study period, influenza vaccination programs in five South American countries prevented more than one-third of laboratory confirmed influenza-associated hospitalizations in young children receiving the recommended two doses and vaccinated older adults.

Dose-Dependent Negative Effects of Prior Multiple Vaccinations Against Influenza A and Influenza B Among Schoolchildren: A Study of Kamigoto Island in Japan During the 2011-2012, 2012-2013, and 2013-2014 Influenza Seasons

Background

We investigated the negative effects of prior multiple vaccinations on influenza vaccine effectiveness (VE) and analyzed the association of VE with prior vaccine doses.

Methods

Patients aged 9-18 years presenting with influenza-like illness at a community hospital on a remote Japanese island during the 2011-2012, 2012-2013, and 2013-2014 influenza seasons were tested for influenza using a rapid diagnostic test (RDT). A test-negative, case-control study design was used to estimate the VEs of trivalent inactivated influenza vaccine. Histories of vaccination and medically attended influenza (MA-flu) A and B during 3 previous seasons were collected from registry systems. VE was calculated using multilevel mixed-effects logistic regression models adjusted for the history of RDT-confirmed MA-flu.

Results

During 3 influenza seasons, 1668 influenza-like illness episodes were analyzed, including 421 and 358 episodes of MA-fluA and MA-fluB, respectively. The adjusted VE (95% confidence interval) yielded significant dose-dependent attenuations by prior vaccinations against both MA-fluA (0 doses during previous 3 seasons: 96% [69%-100%], 1 dose: 48% [-7% to 74%], 2 doses: 52% [11%-74%], 3 doses: 21% [-25% to 51%]; P for trend < .05) and MA-fluB (0 doses: 66% [-5% to 89%], 1 dose: 48% [-14% to 76%], 2 doses: 34% [-33% to 67%], 3 doses: -7% [-83% to 37%]; P for trend < .05). After excluding episodes of MA-flu during prior 3 seasons, similar trends were observed.

Conclusions

Repeated previous vaccinations over multiple seasons had significant dose-dependent negative impacts on VE against both MA-fluA and MA-fluB. Further studies to confirm this finding are necessary.

Effectiveness of Partial and Full Influenza Vaccination Among Children Aged <9 Years in Hong Kong, 2011-2019

BACKGROUND

Two doses of influenza vaccination are recommended for previously unvaccinated children aged <9 years, and receipt of 1 dose is sometimes termed "partial vaccination." We assessed the effectiveness of partial and full influenza vaccination in preventing influenza-associated hospitalization among children in Hong Kong.

METHODS

Using the test-negative design we enrolled 23 187 children aged <9 years admitted to hospitals with acute respiratory illness from September 2011 through March 2019. Vaccination and influenza status were recorded. Fully vaccinated children included those vaccinated with 2 doses or, if previously vaccinated, those vaccinated with 1 dose. Partially vaccinated children included those who should have received 2 doses but only received 1 dose. We estimated vaccine effectiveness (VE) by using conditional logistic regression models matched on epidemiological week.

RESULTS

Overall VE estimates among fully and partially vaccinated children were 73% (95% confidence interval, 69%-77%) and 31% (95% confidence interval, 8%-48%), respectively. A consistently higher VE was observed in children fully vaccinated against each influenza virus type/subtype. The effectiveness of partial vaccination did not vary by age group.

CONCLUSIONS

Partial vaccination was significantly less effective than full vaccination. Our study supports the current recommendation of 2 doses of influenza vaccination in previously unvaccinated children <9 years of age.

Dose effect of influenza vaccine on protection against laboratory-confirmed influenza illness among children aged 6 months to 8 years of age in southern China, 2013/14-2015/16 seasons: a matched case-control study

Background We conducted a matched case-control study in China during the 2013/14-2015/16 influenza seasons to estimate influenza vaccine effectiveness (VE) by dose among children aged 6 months to 8 years.Methods Cases were laboratory-confirmed influenza infections identified through the influenza-like illness sentinel surveillance network in Guangzhou. Age- and sex-matched community controls were randomly selected through the expanded immunization program database. We defined priming as receipt of ≥1 dose of influenza vaccine during the immediate prior season.Results In total, 4,185 case-control pairs were analyzed. Among children 6-35 months, VE for current season dose(s) across the three seasons during 2013/14-2015/16 were 59% (95% Confidence Interval: 44-71%), 12% (-11%,30%), 54% (32-69%); among unprimed children 6-35 months, VE for 1 vs 2 current season doses were 45% (8-67%) vs 65% (46-78%), -2% (-53%,32%) vs 19% (-11%,40%), and 37% (-24%,68%) vs 61% (32-78%). Among children aged 3-8 years, VE for current season dose(s) across study seasons were 62% (36-78%), 43% (22-58%), 32% (1-53%). VE for unprimed children receiving 1 dose only in current season was insignificant or lower than among all children.Conclusion Findings support utility of providing second dose ("booster dose") of seasonal influenza vaccine to unprimed children aged 6-35 months, and the need to study further dose effect of a booster dose among unprimed children aged 3-8 years in China.

Influenza Vaccination: Effectiveness, Indications, and Limits in the Pediatric Population

Influenza vaccine is considered the most effective way to prevent influenza. Nonetheless, every year vaccine coverage is lower than recommended in the pediatric population. Many factors are supposed to contribute to this phenomenon such as the uncertainty about the indication for vaccination, and the suboptimal vaccine-effectiveness in pediatric age, especially in the youngest children. In this review we discuss the effectiveness, indications, and limits of influenza vaccination in the pediatric population based on the most recent evidences.

The impact of repeated vaccination on influenza vaccine effectiveness: a systematic review and meta-analysis

BACKGROUND

Conflicting results regarding the impact of repeated vaccination on influenza vaccine effectiveness (VE) may cause confusion regarding the benefits of receiving the current season's vaccine.

METHODS

We systematically searched MEDLINE, Embase, PubMed, and Cumulative Index to Nursing and Allied Health Literature from database inception to August 17, 2016, for observational studies published in English that reported VE against laboratory-confirmed influenza for the following four vaccination groups: current season only, prior season only, both seasons, and neither season. We pooled differences in VE (∆VE) between vaccination groups by influenza season and type/subtype using a random-effects model. The study protocol is registered with PROSPERO (registration number: CRD42016037241).

RESULTS

We identified 3435 unique articles, reviewed the full text of 634, and included 20 for meta-analysis. Compared to prior season vaccination only, vaccination in both seasons was associated with greater protection against influenza H1N1 (∆VE = 25%; 95% CI 14%, 35%) and B (∆VE = 18%; 95% CI 3%, 33%), but not H3N2 (∆VE = 7%; 95% CI - 7%, 21%). Compared to no vaccination for either season, individuals who received the current season's vaccine had greater protection against H1N1 (∆VE = 62%; 95% CI 51%, 70%), H3N2 (∆VE = 45%; 95% CI 35%, 53%), and B (∆VE = 64%; 95% CI 57%, 71%). We observed no differences in VE between vaccination in both seasons and the current season only for H1N1 (∆VE = 3%; 95% CI - 8%, 13%), but less protection against influenza H3N2 (∆VE = - 20%; 95% CI - 36%, - 4%), and B (∆VE = - 11%; 95% CI - 20%, - 2%).

CONCLUSIONS

Our results support current season vaccination regardless of prior season vaccination because VE for vaccination in the current season only is higher compared to no vaccination in either season for all types/subtypes, and for H1N1 and influenza B, vaccination in both seasons provides better VE than vaccination in the prior season only. Although VE was lower against H3N2 and B for individuals vaccinated in both seasons compared to those vaccinated in the current season only, it should be noted that past vaccination history cannot be altered and this comparison disregards susceptibility to influenza during the prior season among those vaccinated in the current season only. In addition, our results for H3N2 were particularly influenced by the 2014-2015 influenza season and the impact of repeated vaccination for all types/subtypes may vary from season to season. It is important that future VE studies include vaccination history over multiple seasons to evaluate repeated vaccination in more detail.

Impact of Repeated Influenza Immunization on Respiratory Illness in Children With Preexisting Medical Conditions

PURPOSE

Annual influenza immunization in medical risk groups is recommended in many countries. Recent evidence suggests that repeated inactivated influenza vaccine (IIV) immunization throughout childhood may impair long-term immunity against influenza. We assessed whether prior immunization altered the effect of IIV in children with preexisting medical conditions on primary care-diagnosed respiratory illness (RI) episodes during the influenza season.

METHODS

Electronic records of IIV-immunized children who met the criteria for annual IIV immunization according to Dutch guidelines were extracted from a primary care database from 2004 to 2015. For each year, we collected information on IIV immunization status, primary care-attended RI episodes (including influenza-like illness, acute RI, and asthma exacerbation), and potential confounders. Generalized estimating equations were used to model the association between prior IIV and occurrence of at least one RI episode during the influenza season, with "current year immunized but without IIV history" as reference group.

RESULTS

A total of 4,183 children (follow-up duration: 11,493 child-years) were IIV immunized at least once. Adjusted estimates showed lower odds for RI in current year-immunized children with prior IIV compared with those without (odds ratio [OR] = 0.61; 95% CI, 0.47-0.78 for "current year immunized and one IIV in previous 2 years"; OR = 0.85; 95% CI, 0.68-1.07 for "current year immunized and ≥2 IIVs in previous 3 years, including prior year").

CONCLUSION

Repeated IIV immunization in children with preexisting medical conditions has no negative impact on, and may even increase, long-term protection against RI episodes diagnosed during the influenza season in primary care.

Seasonal influenza vaccine effectiveness against laboratory-confirmed influenza hospitalizations - Latin America, 2013

BACKGROUND

Despite widespread utilization of influenza vaccines, effectiveness (VE) has not been routinely measured in Latin America.

METHODS

We used a case test-negative control design to estimate trivalent inactivated influenza VE against laboratory-confirmed influenza among hospitalized children aged 6months-5years and adults aged ≥60years which are age-groups targeted for vaccination. We sought persons with severe acute respiratory infections (SARI), hospitalized at 71 sentinel hospitals in Argentina, Brazil, Chile, Colombia, Costa Rica, El Salvador, Honduras, Panama, and Paraguay during January-December 2013. Cases had an influenza virus infection confirmed by real-time reverse transcription PCR (rRT-PCR); controls had a negative rRT-PCR result for influenza viruses. We used a two-stage random effects model to estimate pooled VE per target age-group, adjusting for the month of illness onset, age and preexisting medical conditions.

RESULTS

We identified 2620 SARI patients across sites: 246 influenza cases and 720 influenza-negative controls aged ≤5years and 448 cases and 1206 controls aged ≥60years. The most commonly identified subtype among participants (48%) was the influenza A(H1N1)pdm09 virus followed by influenza A(H3N2) (34%) and influenza B (18%) viruses. Among children, the adjusted VE of full vaccination (one dose for previously vaccinated or two if vaccine naïve) against any influenza virus SARI was 47% (95% confidence interval [CI]: 14-71%); VE was 58% (95% CI: 16-79%) against influenza A(H1N1)pdm09, and 65% (95% CI: -9; 89%) against influenza A(H3N2) viruses associated SARI. Crude VE of full vaccination against influenza B viruses associated SARI among children was 3% (95% CI: -150; 63). Among adults aged ≥60years, adjusted VE against any influenza SARI was 48% (95% CI: 34-60%); VE was 54% (95% CI: 37-69%) against influenza A(H1N1)pdm09, 43% (95% CI: 18-61%) against influenza A(H3N2) and 34% (95% CI: -4; 58%) against B viruses associated SARI.

CONCLUSION

Influenza vaccine provided moderate protection against severe influenza illness among fully vaccinated young children and older adults, supporting current vaccination strategies.

Potential impact of B lineage mismatch on trivalent influenza vaccine effectiveness during the 2015-2016 influenza season among nursery school children in Suzhou, China

BACKGROUND

We actively followed a cohort of nursery school children in Suzhou, China to assess the impact of vaccination with trivalent influenza vaccine on the prevention of influenza like illness (ILI).

METHODS

We enrolled children aged 36 to 72 months from 13 nursery schools in Suzhou starting two weeks after vaccination during October 2015-February 2016. Every school-day, teachers reported the names of students with ILI to study clinicians, who collected the student's nasopharyngeal swab or throat swab, either at a study clinic or the child's home. Swabs were sent to the Suzhou Center for Disease Control and Prevention's laboratory for influenza testing by RT-PCR.

RESULTS

In total, 3278 children were enrolled; 83 (3%) were lost to follow-up, while 3195 (vaccinated: 1492, unvaccinated: 1703) were followed for 24 weeks. During the study, 40 samples tested positive; 17 in the vaccinated (B Victoria: 12; A(H1N1)pdm09: 5) and 23 in the unvaccinated group (B Victoria: 10; B Yamagata: 2; A(H1N1)pdm09: 11). The VE estimates were: 16% overall (95%CI:-58%,56%), 48% (-47%,84%) for influenza A(H1N1)pdm09, 43% (-650%,98%) for influenza B Yamagata, and -37% (-227%,42%) for influenza B Victoria. Data were analyzed by vaccinated and unvaccinated groups based on enrollees' vaccination records.

CONCLUSIONS

The VE for A(H1N1)pdm09 was moderate but not significant. Mismatching of B lineage may have compromised trivalent influenza vaccine effectiveness during the 2015-2016 influenza season among nursery school children in Suzhou, China. Additional larger studies are warranted to inform policy related to quadrivalent influenza vaccine licensure in China in the future.

Influenza immunization policies: Which could be the main reasons for differences among countries?

Despite the availability of effective prophylactic and therapeutic measures, influenza remains one of the most important infectious disease threats to the human population. Every year, seasonal influenza epidemics infect up to 30% of the population; a relevant portion of the ill are hospitalized, and more than a marginal number die. In an attempt to reduce the medical, social and economic burden of influenza, vaccines are recommended by many health authorities worldwide. However, not all countries have a national program for influenza immunization. The main aim of this paper is to list the differences among influenza immunization policies of various countries, highlighting the most important scientific reasons that may have led health authorities to make different decisions. The manuscript highlights that national influenza immunization policies can vary significantly from country to country. These differences arise from insufficient evidence of the relevance of influenza infection from a clinical, social and economic point of view. The lack of precise data on the true frequency and clinical relevance of influenza infection makes it nearly impossible to establish the economic burden of influenza. Moreover, it remains very difficult to evaluate the efficacy of the different influenza vaccines and whether their use is cost-effective considering the various types of people receiving them and the indirect advantages. Disparities among countries will be overcome only when more reliable data regarding all these aspects of influenza infection, particularly those related to the true impact of the disease, are precisely defined.

Vaccine effectiveness against laboratory-confirmed influenza hospitalizations among young children during the 2010-11 to 2013-14 influenza seasons in Ontario, Canada

Uncertainty remains regarding the magnitude of effectiveness of influenza vaccines for preventing serious outcomes, especially among young children. We estimated vaccine effectiveness (VE) against laboratory-confirmed influenza hospitalizations among children aged 6-59 months. We used the test-negative design in hospitalized children in Ontario, Canada during the 2010-11 to 2013-14 influenza seasons. We used logistic regression models adjusted for age, season, and time within season to calculate VE estimates by vaccination status (full vs. partial), age group, and influenza season. We also assessed VE incorporating prior history of influenza vaccination. We included specimens from 9,982 patient hospitalization episodes over four seasons, with 12.8% testing positive for influenza. We observed variation in VE by vaccination status, age group, and influenza season. For the four seasons combined, VE was 60% (95%CI, 44%-72%) for full vaccination and 39% (95%CI, 17%-56%) for partial vaccination. VE for full vaccination was 67% (95%CI, 48%-79%) for children aged 24-59 months, 48% (95%CI, 12%-69%) for children aged 6-23 months, 77% (95%CI, 47%-90%) for 2010-11, 59% (95%CI, 13%-81%) for 2011-12, 33% (95%CI, -18% to 62%) for 2012-13, and 72% (95%CI, 42%-86%) for 2013-14. VE in children aged 24-59 months appeared similar between those vaccinated in both the current and previous seasons and those vaccinated in the current season only, with the exception of 2012-13, when VE was lower for those vaccinated in the current season only. Influenza vaccination is effective in preventing pediatric laboratory-confirmed influenza hospitalizations during most seasons.

Influenza vaccine use to protect healthy children: A debated topic

At the beginning of this century, a number of studies suggested that in healthy children, particularly those <2years of age, influenza could have a serious and complicated course, as it frequently led to hospitalization and sometimes, albeit rarely, to death. Moreover, pre-schoolers and school-age children were found to be among the most important causes of influenza transmission to the community, as they shed the virus for a longer time than adults and had frequent contact with greater numbers of individuals through day-care and school. These findings led a number of health authorities to modify the official recommendations regarding the use of influenza vaccine in healthy children. Several factors seem to indicate that vaccination against influenza in healthy children of any age and in pregnant women could be effective in preventing the disease in the entire paediatric population and in providing herd immunity in adults and old people as well. The direct advantages of the vaccine seem greater in younger subjects, particularly those <2-3years of age. Vaccination of older children is considered effective by most experts, but high vaccination coverage of these subjects has been difficult to attain. Similar difficulties have been identified for the vaccination of pregnant women. These challenges can be overcome, at least in part, by appropriate information and accurate evaluations of available data. In addition, further studies specifically designed to clarify unresolved problems regarding vaccine use in paediatric and pregnant populations are needed to convince reluctant health authorities. More effective vaccines for younger children as well as improved availability of data regarding the optimal time period for vaccine administration in pregnant women appear relevant in this regard.

The impact of repeated vaccination on influenza vaccine effectiveness: a systematic review and meta-analysis

BACKGROUND

Conflicting results regarding the impact of repeated vaccination on influenza vaccine effectiveness (VE) may cause confusion regarding the benefits of receiving the current season's vaccine.

METHODS

We systematically searched MEDLINE, Embase, PubMed, and Cumulative Index to Nursing and Allied Health Literature from database inception to August 17, 2016, for observational studies published in English that reported VE against laboratory-confirmed influenza for four vaccination groups, namely current season only, prior season only, both seasons, and neither season. We pooled differences in VE (∆VE) between vaccination groups by influenza season and type/subtype using a random effects model. The study protocol is registered with PROSPERO (registration number: CRD42016037241).

RESULTS

We identified 3435 unique articles, reviewed the full text of 634, and included 20 for meta-analysis. Compared to prior season vaccination only, vaccination in both seasons was associated with greater protection against influenza H1N1 (∆VE = 26%; 95% CI, 15% to 36%) and B (∆VE = 24%; 95% CI, 7% to 42%), but not H3N2 (∆VE = 10%; 95% CI, -6% to 25%). Compared to no vaccination for either season, individuals who received the current season's vaccine had greater protection against H1N1 (∆VE = 61%; 95% CI, 50% to 70%), H3N2 (∆VE = 41%; 95% CI, 33% to 48%), and B (∆VE = 62%; 95% CI, 54% to 68%). We observed no differences in VE between vaccination in both seasons and the current season only for H1N1 (∆VE = 4%; 95% CI, -7% to 15%), H3N2 (∆VE = -12%; 95% CI, -27% to 4%), or B (∆VE = -8%; 95% CI, -17% to 1%).

CONCLUSIONS

From the patient perspective, our results support current season vaccination regardless of prior season vaccination. We found no overall evidence that prior season vaccination negatively impacts current season VE. It is important that future VE studies include vaccination history over multiple seasons in order to evaluate repeated vaccination in more detail.

Repeated annual influenza vaccination and vaccine effectiveness: review of evidence

INTRODUCTION

Studies in the 1970s and 1980s signaled concern that repeated influenza vaccination could affect vaccine protection. The antigenic distance hypothesis provided a theoretical framework to explain variability in repeat vaccination effects based on antigenic similarity between successive vaccine components and the epidemic strain. Areas covered: A meta-analysis of vaccine effectiveness studies from 2010-11 through 2014-15 shows substantial heterogeneity in repeat vaccination effects within and between seasons and subtypes. When negative effects were observed, they were most pronounced for H3N2, especially in 2014-15 when vaccine components were unchanged and antigenically distinct from the epidemic strain. Studies of repeated vaccination across multiple seasons suggest that vaccine effectiveness may be influenced by more than one prior season. In immunogenicity studies, repeated vaccination blunts the hemagglutinin antibody response, particularly for H3N2. Expert commentary: Substantial heterogeneity in repeated vaccination effects is not surprising given the variation in study populations and seasons, and the variable effects of antigenic distance and immunological landscape in different age groups and populations. Caution is required in the interpretation of pooled results across multiple seasons, since this can mask important variation in repeat vaccination effects between seasons. Multi-season clinical studies are needed to understand repeat vaccination effects and guide recommendations.

Influenza Vaccine Effectiveness in Preventing Influenza Illness Among Children During School-based Outbreaks in the 2014-2015 Season in Beijing, China

BACKGROUND

Little is known about vaccine effectiveness (VE) against nonmedically attended A(H3N2) influenza illness during 2014-2015 when the vaccine component appeared to be a poor match with circulating strains.

METHODS

Forty-three eligible school influenza outbreaks in Beijing, China, from November 1, 2014, to December 31, 2014, were included in this study. The VE of 2014-2015 trivalent inactivated influenza vaccine (IIV3) was assessed in preventing laboratory-confirmed influenza among school-age children through a case-control design, using asymptomatic controls. Influenza vaccination was documented from a vaccination registry. VE was estimated adjusting for age group, sex, rural versus urban area, body mass index, chronic conditions, onset week and schools through a mixed effects logistic regression model.

RESULTS

The average coverage rate of 2014-2015 IIV3 among students across the 43 schools was 47.6%. The fully adjusted VE of 2014-2015 IIV3 against laboratory-confirmed influenza was 38% [95% confidence interval (CI): 12%-57%]. Receipt of previous season's (2013-2014) IIV3 significantly modified VE of the 2014-2015 IIV3; children who received both 2013-2014 and 2014-2015 vaccinations had VE of 29% (95% CI: -8% to 53%), whereas VE for children who received 2014-2015 IIV3 only was 54% (95% CI: 8%-77%).

CONCLUSIONS

VE for 2014-2015 IIV3 against A(H3N2) illness identified in schools was modest. Children who did not receive the prior season's vaccine with a homologous A(H3N2) component may have enjoyed greater protection than repeated vaccinees.

Influenza Vaccine Effectiveness for Fully and Partially Vaccinated Children 6 Months to 8 Years Old During 2011-2012 and 2012-2013: The Importance of Two Priming Doses

BACKGROUND

Few studies have examined the effectiveness of full versus partial vaccination with inactivated trivalent influenza vaccines (IIV3) as defined by the US CDC Advisory Committee on Immunization Practices.

METHODS

Respiratory swabs were collected from outpatients aged 6 months to 8 years with acute cough for ≤7 days in clinics in 5 states during the 2011-2012 and 2012-2013 influenza seasons. Influenza was confirmed by real-time reverse transcription polymerase chain reaction assay. Receipt of current season IIV3 and up to 4 prior vaccinations was documented from medical records and immunization registries. Using a test-negative design, vaccine effectiveness (VE) was estimated adjusting for age, race/ethnicity, medical conditions, study site and month of enrollment.

RESULTS

We did not observe higher VE for children fully versus partially vaccinated with IIV3, as defined by US Advisory Committee on Immunization Practice, although our sample of partially vaccinated children was relatively small. However, among children aged 2-8 years in both seasons and against A(H3N2) and B influenza illness separately, VE point estimates were consistently higher for children who had received 2 doses in the same prior season compared with those without (VE range of 58%-80% vs. 33%-44%, respectively). Across seasons, the odds of A(H3N2) illness despite IIV3 vaccination were 2.4-fold (95% confidence interval: 1.4-4.3) higher among children who had not received 2 doses in the same prior season. We also noted residual protection among unvaccinated children who were vaccinated the previous season (VE range = 36%-40% across outcomes).

CONCLUSION

Vaccination with IIV3 may provide preventive benefit in subsequent seasons, including possible residual protection if vaccination is missed. Two vaccine doses in the same season may be more effective than alternative priming strategies.

Effectiveness of subunit influenza vaccination in the 2014-2015 season and residual effect of split vaccination in previous seasons

BACKGROUND

In Navarra, Spain, subunit vaccine was first used in the 2014-2015 season, whereas trivalent split-virion influenza vaccines had been used in previous seasons. We estimate the effectiveness of the subunit vaccine in the current season and split vaccine in the two previous seasons against laboratory-confirmed influenza in the 2014-2015 season.

METHODS

Patients with influenza-like illness hospitalized or attended by sentinel general practitioners were swabbed for influenza testing. The previous and current vaccine status of laboratory-confirmed cases was compared to test-negative controls.

RESULTS

Among 1213 patients tested, 619 (51%) were confirmed for influenza virus: 52% influenza A(H3N2), 46% influenza B, and 2% A(H1N1)pdm09. The overall effectiveness for subunit vaccination in the current season was 19% (95% confidence interval [CI]: -13 to 42), 2% (95%CI: -47 to 35) against influenza A(H3N2) and 32% (95%CI: -4 to 56) against influenza B. The effectiveness against any influenza was 67% (95%CI: 17-87) for 2012-2013 and 2013-2014 vaccination only, 42% (95%CI: -31 to 74) for 2014-2015 vaccination only, and 38% (95%CI: 8-58) for vaccination in the 2012-2013, 2013-2014 and 2014-2015 seasons. The same estimates against influenza A(H3N2) were 47% (95%CI: -60 to 82), -54% (95%CI: -274 to 37) and 28% (95%CI: -17 to 56), and against influenza B were 82% (95%CI: 19-96), 93% (95%CI: 45-99) and 43% (95%CI: 5-66), respectively.

CONCLUSION

These results suggest a considerable residual protection of split vaccination in previous seasons, low overall effectiveness of current season subunit vaccination, and possible interference between current subunit and previous split vaccines.