National Influenza Annual Report, Canada, 2022-2023: Canada's first fall epidemic since the 2019-2020 season

Coinciding with the beginning of the coronavirus disease 2019 (COVID-19) pandemic in March 2020, Canadian seasonal influenza circulation was suppressed, which was a trend reported globally. Canada saw a brief and delayed return of community influenza circulation during the spring of the 2021-2022 influenza season. Surveillance for Canada's 2022-2023 seasonal influenza epidemic began in epidemiological week 35 (week starting August 28, 2022) and ended in epidemiological week 34 (week ending August 26, 2023). The 2022-2023 season marked the return to pre-pandemic-like influenza circulation. The epidemic began in epidemiological week 43 (week ending October 29, 2022) and lasted 10 weeks. Driven by influenza A(H3N2), the epidemic was relatively early, extraordinary in intensity, and short in length. This season, a total of 74,344 laboratory-confirmed influenza detections were reported out of 1,188,962 total laboratory tests. A total of 93% of detections were influenza A (n=68,923). Influenza A(H3N2) accounted for 89% of the subtyped specimens (n=17,638/19,876). Late-season, Canada saw community circulation of influenza B for the first time since the 2019-2020 season. The 2022-2023 influenza season in Canada had an extraordinary impact on children and youth; nearly half (n=6,194/13,729, 45%) of reported influenza A(H3N2) detections were in the paediatric (younger than 19 years) population. Weekly paediatric influenza-associated hospital admissions were persistently above historical peak levels for several weeks. The total number of influenza-associated paediatric hospitalizations (n=1,792) far exceeded historical averages (n=1,091). With the return of seasonal influenza circulation and endemic co-circulation of multiple high burden respiratory viruses, sustained vigilance is warranted. Annual seasonal influenza vaccination is a key public health intervention available to protect Canadians.

A large A(H3N2) influenza outbreak with a high attack rate in a drug user community in Italy, April 2022

Despite the COVID-19 pandemic, influenza remains an important issue. Especially in community settings, influenza outbreaks can be difficult to control and can result in high attack rates. In April 2022, a large A(H3N2) influenza outbreak spread in the largest Italian drug-rehabilitation community. One hundred eighty-four individuals presented influenza-like symptoms (attack rate of 26.2%); 56% previously received the influenza vaccine. Sequence analyses highlighted a genetic drift from the vaccine strain, which may have caused the observed lack of protection.

Successful Treatment of Complicated Influenza A(H3N2) Virus Infection and Rhabdomyolysis with Compassionate Use of IV Zanamivir

In 2019, EMA licensed intravenous (IV) zanamivir for severe influenza virus infection in children over 6 months as well as adults. Prior to that, it was possible via a compassionate use program. We present successful compassionate use of IV zanamivir in a 14-year-old female with severe influenza A(H3N2) and multi-organ failure, who had failed oral oseltamivir. Her illness was complicated by acute respiratory distress syndrome and rhabdomyolysis requiring extracorporeal membrane oxygenation and hemofiltration. Considering the broad safety margins with neuraminidase inhibitors, an adult dose of 600 mg IV BID was administered in this 60 kg patient. Influenza virus was cleared rapidly and undetectable on day 13. Creatine kinase (CK) values were dropping from 38,000 to 500 within nine days. Given the recent licensure of IV zanamivir, multi-center prospective observational studies in pediatric Intensive Care Unit patients would be beneficial to guide the most appropriate use of IV zanamivir in this vulnerable age group.

National influenza mid-season report, 2022-2023: A rapid and early epidemic onset

Canada's 2022-2023 national influenza epidemic was declared in epidemiological week 43 (week ending October 29, 2022), relatively early in comparison to historical seasons. This year marks the return to pre-pandemic-like influenza circulation, following the brief and delayed influenza epidemic declared in the spring of the 2021-2022 season. To date this season, 59,459 detections of influenza have been reported out of 456,536 tests; both values exceeding historical averages. This epidemic is being fundamentally driven by influenza A, with influenza A(H3N2) accounting for 94% of subtyped detections. This season to date has had a significant impact on adolescents and young children, with a high proportion of detections occurring in those aged 0-19 years (42%). Provinces and territories have reported higher than usual influenza-associated hospitalizations, intensive care unit admissions, and deaths in comparison with previous seasons; in particular, paediatric hospitalization incidence was persistently far above historical peak levels for several weeks. The return of seasonal influenza circulation highlights the importance of sustained vigilance with regard to influenza and employment of available mitigation measures, especially of annual seasonal influenza vaccination.

Illness Severity in Hospitalized Influenza Patients by Virus Type and Subtype, Spain, 2010-2017

Influenza A(H1N1)pdm09 caused more hospitalizations, intensive care unit admissions, and deaths than influenza A(H3N2) or B.

We conducted a retrospective cohort study to assess the effect of influenza virus type and subtype on disease severity among hospitalized influenza patients in Spain. We analyzed the cases of 8,985 laboratory-confirmed case-patients hospitalized for severe influenza by using data from a national surveillance system for the period 2010–2017. Hospitalized patients with influenza A(H1N1)pdm09 virus were significantly younger, more frequently had class III obesity, and had a higher risk for pneumonia or acute respiratory distress syndrome than patients infected with influenza A(H3N2) or B (p<0.05). Hospitalized patients with influenza A(H1N1)pdm09 also had a higher risk for intensive care unit admission, death, or both than patients with influenza A(H3N2) or B, independent of other factors. Determining the patterns of influenza-associated severity and how they might differ by virus type and subtype can help guide planning and implementation of adequate control and preventive measures during influenza epidemics.

Comparison of the Outcomes of Individuals With Medically Attended Influenza A and B Virus Infections Enrolled in 2 International Cohort Studies Over a 6-Year Period: 2009-2015

Background

Outcome data from prospective follow-up studies comparing infections with different influenza virus types/subtypes are limited.

Methods

Demographic, clinical characteristics and follow-up outcomes for adults with laboratory-confirmed influenza A(H1N1)pdm09, A(H3N2), or B virus infections were compared in 2 prospective cohorts enrolled globally from 2009 through 2015. Logistic regression was used to compare outcomes among influenza virus type/subtypes.

Results

Of 3952 outpatients, 1290 (32.6%) had A(H1N1)pdm09 virus infection, 1857 (47.0%) had A(H3N2), and 805 (20.4%) had influenza B. Of 1398 inpatients, 641 (45.8%) had A(H1N1)pdm09, 532 (38.1%) had A(H3N2), and 225 (16.1%) had influenza B. Outpatients with A(H1N1)pdm09 were younger with fewer comorbidities and were more likely to be hospitalized during the 14-day follow-up (3.3%) than influenza B (2.2%) or A(H3N2) (0.7%; P < .0001). Hospitalized patients with A(H1N1)pdm09 (20.3%) were more likely to be enrolled from intensive care units (ICUs) than those with A(H3N2) (11.3%) or B (9.8%; P < .0001). However, 60-day follow-up of discharged inpatients showed no difference in disease progression (P = .32) or all-cause mortality (P = .30) among influenza types/subtypes. These findings were consistent after covariate adjustment, in sensitivity analyses, and for subgroups defined by age, enrollment location, and comorbidities.

Conclusions

Outpatients infected with influenza A(H1N1)pdm09 or influenza B were more likely to be hospitalized than those with A(H3N2). Hospitalized patients infected with A(H1N1)pdm09 were younger and more likely to have severe disease at study entry (measured by ICU enrollment), but did not have worse 60-day outcomes.

Molecular Dynamics Simulation of the Influenza A(H3N2) Hemagglutinin Trimer Reveals the Structural Basis for Adaptive Evolution of the Recent Epidemic Clade 3C.2a

Influenza A(H3N2) has been a major cause of seasonal influenza in humans since 1968, and has evolved by antigenic drift under the constantly changing human herd immunity. Increasing evidence suggests that the antigenic change occasionally occurred concomitant with the alterations of the N-glycosylation site profile and hemagglutination activity of the virion surface protein hemagglutinin (HA). However, the structural basis of these changes remains largely unclear. To address this issue, we performed molecular dynamics simulations of the glycosylated HA trimers of the A(H3N2), which has a novel pattern of Asn-X-Ser/Thr sequons unique in the new A(H3N2) epidemic clade 3C.2a and is characterized by attenuated ability to agglutinate nonhuman erythrocytes. Comparison of the equilibrated structures of the glycosylated HA trimers with and without the 3C.2a-specific mutations reveals that the mutations could induce a drastic reduction in the apical space for the ligand binding via glycan-shield rearrangement. The results suggest that the 3C.2a strain has evolved an HA structure that is advantageous for evading pre-existing antibodies, while also increasing the ligand binding specificity. These findings have structural implications for our understanding of the phenotypic changes, evolution, and fate of influenza A(H3N2).

Pandemic vaccination strategies and influenza severe outcomes during the influenza A(H1N1)pdm09 pandemic and the post-pandemic influenza season: the Nordic experience

During the 2009/10 influenza A(H1N1)pdm09 pandemic, the five Nordic countries adopted different approaches to pandemic vaccination. We compared pandemic vaccination strategies and severe influenza outcomes, in seasons 2009/10 and 2010/11 in these countries with similar influenza surveillance systems. We calculated the cumulative pandemic vaccination coverage in 2009/10 and cumulative incidence rates of laboratory confirmed A(H1N1)pdm09 infections, intensive care unit (ICU) admissions and deaths in 2009/10 and 2010/11. We estimated incidence risk ratios (IRR) in a Poisson regression model to compare those indicators between Denmark and the other countries. The vaccination coverage was lower in Denmark (6.1%) compared with Finland (48.2%), Iceland (44.1%), Norway (41.3%) and Sweden (60.0%). In 2009/10 Denmark had a similar cumulative incidence of A(H1N1)pdm09 ICU admissions and deaths compared with the other countries. In 2010/11 Denmark had a significantly higher cumulative incidence of A(H1N1)pdm09 ICU admissions (IRR: 2.4; 95% confidence interval (CI): 1.9-3.0) and deaths (IRR: 8.3; 95% CI: 5.1-13.5). Compared with Denmark, the other countries had higher pandemic vaccination coverage and experienced less A(H1N1)pdm09-related severe outcomes in 2010/11. Pandemic vaccination may have had an impact on severe influenza outcomes in the post-pandemic season. Surveillance of severe outcomes may be used to compare the impact of influenza between seasons and support different vaccination strategies.

Hemagglutinin Gene Clade 3C.2a Influenza A(H3N2) Viruses, Alachua County, Florida, USA, 2014-15

Influenza A(H3N2) strains isolated during 2014-15 in Alachua County, Florida, USA, belonged to hemagglutinin gene clade 3C.2a. High rates of influenza-like illness and confirmed influenza cases in children were associated with a decrease in estimated vaccine effectiveness. Illnesses were milder than in 2013-14; severe cases were concentrated in elderly patients with underlying diseases.

Effectiveness of seasonal influenza vaccinations against laboratory-confirmed influenza-associated infections among Singapore military personnel in 2010-2013

BACKGROUND

Limited information is available about seasonal influenza vaccine effectiveness (VE) in tropical communities.

OBJECTIVES

Virus subtype-specific VE was determined for all military service personnel in the recruit camp and three other non-recruit camp in Singapore's Armed Forces from 1 June 2009 to 30 June 2012.

METHODS

Consenting servicemen underwent nasal washes, which were tested with RT-PCR and subtyped. The test positive case and test negative control design was used to estimate the VE. To estimate the overall effect of the programme on new recruits, we used an ecological time series approach.

RESULTS

A total of 7016 consultations were collected. The crude estimates for the VE of the triavalent vaccine against both influenza A(H1N1)pdm09 and influenza B were 84% (95% CI 78-88%, 79-86%, respectively). Vaccine efficacy against influenza A(H3N2) was markedly lower (VE 33%, 95% CI -4% to 57%). An estimated 70% (RR = 0.30; 95% CI 0.11-0.84), 39% (RR = 0.61;0.25-1.43) and 75% (RR = 0.25; 95% CI 0.11-0.50) reduction in the risk of influenza A(H1N1)pdm09, influenza A(H3N2) and influenza B infections, respectively, in the recruit camp during the post-vaccination period compared with during the pre-vaccination period was observed.

CONCLUSIONS

Overall, the blanket influenza vaccine programme in Singapore's Armed Forces has had a moderate to high degree of protection against influenza A(H1N1)pdm09 and influenza B, but not against influenza A(H3N2). Blanket influenza vaccination is recommended for all military personnel.