SARS-CoV-2 outbreak among staff and evacuees at Operation Allies Welcome Safe Havens

We report on five SARS-CoV-2 congregate setting outbreaks at U.S. Operation Allies Welcome Safe Havens/military facilities. Outbreak data were collected, and attack rates were calculated for various populations. Even in vaccinated populations, there was rapid spread, illustrating the importance of institutional prevention and mitigation policies in congregate settings.

Assessment of Anti-SARS-CoV-2 antibody levels among university students vaccinated with different COVID-19 primary and booster doses - fall 2021, Wisconsin

BACKGROUND

University students commonly received COVID-19 vaccinations before returning to U.S. campuses in the Fall of 2021. Given likely immunologic variation among students based on differences in type of primary series and/or booster dose vaccine received, we conducted serologic investigations in September and December 2021 on a large university campus in Wisconsin to assess anti-SARS-CoV-2 antibody levels.

METHODS

We collected blood samples, demographic information, and COVID-19 illness and vaccination history from a convenience sample of students. Sera were analyzed for both anti-spike (anti-S) and anti-nucleocapsid (anti-N) antibody levels using World Health Organization standardized binding antibody units per milliliter (BAU/mL). Levels were compared across categorical primary COVID-19 vaccine series received and binary COVID-19 mRNA booster status. The association between anti-S levels and time since most recent vaccination dose was estimated by mixed-effects linear regression.

RESULTS

In total, 356 students participated, of whom 219 (61.5%) had received a primary vaccine series of Pfizer-BioNTech or Moderna mRNA vaccines and 85 (23.9%) had received vaccines from Sinovac or Sinopharm. Median anti-S levels were significantly higher for mRNA primary vaccine series recipients (2.90 and 2.86 log [BAU/mL], respectively), compared with those who received Sinopharm or Sinovac vaccines (1.63 and 1.95 log [BAU/mL], respectively). Sinopharm and Sinovac vaccine recipients were associated with a significantly faster anti-S decline over time, compared with mRNA vaccine recipients (P <.001). By December, 48/172 (27.9%) participants reported receiving an mRNA COVID-19 vaccine booster, which reduced the anti-S antibody discrepancies between primary series vaccine types.

CONCLUSIONS

Our work supports the benefit of heterologous boosting against COVID-19. COVID-19 mRNA vaccine booster doses were associated with increases in anti-SARS-CoV-2 antibody levels; following an mRNA booster dose, students with both mRNA and non-mRNA primary series receipt were associated with comparable levels of anti-S IgG.

K-medoids clustering of hospital admission characteristics to classify severity of influenza virus infection

Background

Patients are admitted to the hospital for respiratory illness at different stages of their disease course. It is important to appropriately analyse this heterogeneity in surveillance data to accurately measure disease severity among those hospitalized. The purpose of this study was to determine if unique baseline clusters of influenza patients exist and to examine the association between cluster membership and in-hospital outcomes.

Methods

Patients hospitalized with influenza at two hospitals in Southeast Michigan during the 2017/2018 (n = 242) and 2018/2019 (n = 115) influenza seasons were included. Physiologic and laboratory variables were collected for the first 24 h of the hospital stay. K-medoids clustering was used to determine groups of individuals based on these values. Multivariable linear regression or Firth's logistic regression were used to examine the association between cluster membership and clinical outcomes.

Results

Three clusters were selected for 2017/2018, mainly differentiated by blood glucose level. After adjustment, those in C₁₇1 had 5.6 times the odds of mechanical ventilator use than those in C₁₇2 (95% CI: 1.49, 21.1) and a significantly longer mean hospital length of stay than those in both C₁₇2 (mean 1.5 days longer, 95% CI: 0.2, 2.7) and C₁₇3 (mean 1.4 days longer, 95% CI: 0.3, 2.5). Similar results were seen between the two clusters selected for 2018/2019.

Conclusion

In this study of hospitalized influenza patients, we show that distinct clusters with higher disease acuity can be identified and could be targeted for evaluations of vaccine and influenza antiviral effectiveness against disease attenuation. The association of higher disease acuity with glucose level merits evaluation.

Behaviors and attitudes of college students during an academic semester at two Wisconsin universities during the COVID-19 pandemic

OBJECTIVE

Characterize college student COVID-19 behaviors and attitudes during the early pandemic. Participants: Students on two university campuses in Wisconsin.

METHODS

Surveys administered in September and November 2020.

RESULTS

Few students (3-19%) participated in most in-person activities during the semester, with eating at restaurants as the exception (72-80%) and attending work (35%) and parties (33%) also reported more frequently. The majority wore masks in public (94-99%), but comparatively fewer (42%) did so at parties. Mask-wearing at parties decreased from September to November (p < 0.05). Students attending parties, or consuming more alcohol, were less concerned and more likely to take COVID-19-associated risks.

CONCLUSIONS

Students were motivated to adhere to COVID-19 prevention measures but gathered socially. Though there was frequent public masking, mask-wearing at parties declined in November and may represent pandemic fatigue. High-yield strategies for decreasing viral spread may include changing masking social norms and engaging with students about creative risk-reduction strategies.

Public Health Actions to Control Measles Among Afghan Evacuees During Operation Allies Welcome - United States, September-November 2021

On August 29, 2021, the United States government oversaw the emergent establishment of Operation Allies Welcome (OAW), led by the U.S. Department of Homeland Security (DHS) and implemented by the U.S. Department of Defense (DoD) and U.S. Department of State (DoS), to safely resettle U.S. citizens and Afghan nationals from Afghanistan to the United States. Evacuees were temporarily housed at several overseas locations in Europe and Asia* before being transported via military and charter flights through two U.S. international airports, and onward to eight U.S. military bases,† with hotel A used for isolation and quarantine of persons with or exposed to certain infectious diseases.§ On August 30, CDC issued an Epi-X notice encouraging public health officials to maintain vigilance for measles among Afghan evacuees because of an ongoing measles outbreak in Afghanistan (25,988 clinical cases reported nationwide during January-November 2021) (1) and low routine measles vaccination coverage (66% and 43% for the first and second doses, respectively, in 2020) (2).

Antigen Test Performance Among Children and Adults at a SARS-CoV-2 Community Testing Site

BACKGROUND

Performance characteristics of SARS-CoV-2 antigen tests among children are limited despite the need for point-of-care testing in school and childcare settings. We describe children seeking SARS-CoV-2 testing at a community site and compare antigen test performance to real-time reverse transcription-polymerase chain reaction (RT-PCR) and viral culture.

METHODS

Two anterior nasal specimens were self-collected for BinaxNOW antigen and RT-PCR testing, along with demographics, symptoms, and exposure information from individuals ≥5 years at a community testing site. Viral culture was attempted on residual antigen or RT-PCR-positive specimens. Demographic and clinical characteristics, and the performance of SARS-CoV-2 antigen tests, were compared among children (<18 years) and adults.

RESULTS

About 1 in 10 included specimens were from children (225/2110); 16.4% (37/225) were RT-PCR-positive. Cycle threshold values were similar among RT-PCR-positive specimens from children and adults (22.5 vs 21.3, P = .46) and among specimens from symptomatic and asymptomatic children (22.5 vs 23.2, P = .39). Sensitivity of antigen test compared to RT-PCR was 73.0% (27/37) among specimens from children and 80.8% (240/297) among specimens from adults; among specimens from children, specificity was 100% (188/188), positive and negative predictive values were 100% (27/27) and 94.9% (188/198), respectively. Virus was isolated from 51.4% (19/37) of RT-PCR-positive pediatric specimens; all 19 had positive antigen test results.

CONCLUSIONS

With lower sensitivity relative to RT-PCR, antigen tests may not diagnose all positive COVID-19 cases; however, antigen testing identified children with live SARS-CoV-2 virus.

Distinct influenza surveillance networks and their agreement in recording regional influenza circulation: Experience from Southeast Michigan

INTRODUCTION

In Southeast Michigan, active surveillance studies monitor influenza activity in hospitals, ambulatory clinics, and community households. Across five respiratory seasons, we assessed the contribution of data from each of the three networks towards improving our overall understanding of regional influenza circulation.

METHODS

All three networks used case definitions for acute respiratory illness (ARI) and molecularly tested for influenza from research-collected respiratory specimens. Age- and network-stratified epidemic curves were created for influenza A and B. We compared stratified epidemic curves visually and by centering at seasonal midpoints.

RESULTS

Across all seasons (from 2014/2015 through 2018/2019), epidemic curves from each of the three networks were comparable in terms of both timing and magnitude. Small discrepancies in epidemics recorded by each network support previous conclusions about broader characteristics of particular influenza seasons.

CONCLUSION

Influenza surveillance systems based in hospital, ambulatory clinic, and community household settings appear to provide largely similar information regarding regional epidemic activity. Together, multiple levels of influenza surveillance provide a detailed view of regional influenza epidemics, but a single surveillance system-regardless of population subgroup monitored-appears to be sufficient in providing vital information regarding community influenza epidemics.

Interventions to Disrupt Coronavirus Disease Transmission at a University, Wisconsin, USA, August-October 2020

University settings have demonstrated potential for coronavirus disease (COVID-19) outbreaks; they combine congregate living, substantial social activity, and a young population predisposed to mild illness. Using genomic and epidemiologic data, we describe a COVID-19 outbreak at the University of Wisconsin-Madison, Madison, Wisconsin, USA. During August-October 2020, a total of 3,485 students, including 856/6,162 students living in dormitories, tested positive. Case counts began rising during move-in week, August 25-31, 2020, then rose rapidly during September 1-11, 2020. The university initiated multiple prevention efforts, including quarantining 2 dormitories; a subsequent decline in cases was observed. Genomic surveillance of cases from Dane County, in which the university is located, did not find evidence of transmission from a large cluster of cases in the 2 quarantined dorms during the outbreak. Coordinated implementation of prevention measures can reduce COVID-19 spread in university settings and may limit spillover to the surrounding community.

Association of Shared Living Spaces and COVID-19 in University Students, Wisconsin, USA, 2020

We describe characteristics associated with having coronavirus disease (COVID-19) among students residing on a university campus. Of 2,187 students, 528 (24.1%) received a COVID-19 diagnosis during fall semester 2020. Students sharing a bedroom or suite had approximately twice the odds of contracting COVID-19 as those living alone.

Risk Factors for Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Infection and Presence of Anti-SARS-CoV-2 Antibodies Among University Student Dormitory Residents, September-November 2020

BACKGROUND

Multiple severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreaks occurred at universities during Fall 2020, but little is known about risk factors for campus-associated infections or immunity provided by anti-SARS-CoV-2 antibodies in young adults.

METHODS

We conducted surveys and serology tests among students living in dormitories in September and November to examine infection risk factors and antibody presence. Using campus weekly reverse-transcription polymerase chain reaction (RT-PCR) test results, the relationship between survey responses, SARS-CoV-2 antibodies, and infections was assessed.

RESULTS

Of 6136 students, 1197 completed the survey and 572 also completed serologic testing in September compared with 517 and 414 in November, respectively. Participation in fraternity or sorority events (adjusted risk ratio [aRR], 1.9 [95% confidence interval {CI}, 1.4-2.5]) and frequent alcohol consumption (aRR, 1.6 [95% CI, 1.2-2.2]) were associated with SARS-CoV-2 infection. Mask wearing during social events (aRR, 0.6 [95% CI, .6-1.0]) was associated with decreased risk. None of the 20 students with antibodies in September tested positive for SARS-CoV-2 during the semester, while 27.8% of students who tested RT-PCR positive tested negative for antibodies in November.

CONCLUSIONS

Frequent drinking and attending social events were associated with SARS-CoV-2 infection. Antibody presence in September appeared to be protective from reinfection, but this finding was not statistically significant.

Severe Acute Respiratory Syndrome Coronavirus 2 Transmission in Intercollegiate Athletics Not Fully Mitigated With Daily Antigen Testing

Background

High-frequency, rapid-turnaround severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) testing continues to be proposed as a way of efficiently identifying and mitigating transmission in congregate settings. However, 2 SARS-CoV-2 outbreaks occurred among intercollegiate university athletic programs during the fall 2020 semester, despite mandatory directly observed daily antigen testing.

Methods

During the fall 2020 semester, athletes and staff in both programs were tested daily using Quidel’s Sofia SARS Antigen Fluorescent Immunoassay, with positive antigen results requiring confirmatory testing with real-time reverse-transcription polymerase chain reaction. We used genomic sequencing to investigate transmission dynamics in these 2 outbreaks.

Results

In the first outbreak, 32 confirmed cases occurred within a university athletics program after the index patient attended a meeting while infectious, despite a negative antigen test on the day of the meeting. Among isolates sequenced from that outbreak, 24 (92%) of 26 were closely related, suggesting sustained transmission following an initial introduction event. In the second outbreak, 12 confirmed cases occurred among athletes from 2 university programs that faced each other in an athletic competition, despite receipt of negative antigen test results on the day of the competition. Sequences from both teams were closely related and distinct from viruses circulating in the community for team 1, suggesting transmission during intercollegiate competition in the community for team 2.

Conclusions

These findings suggest that antigen testing alone, even when mandated and directly observed, may not be sufficient as an intervention to prevent SARS-CoV-2 outbreaks in congregate settings, and they highlight the importance of vaccination to prevent SARS-CoV-2 outbreak in congregate settings.

Performance of Repeat BinaxNOW Severe Acute Respiratory Syndrome Coronavirus 2 Antigen Testing in a Community Setting, Wisconsin, November 2020-December 2020

Repeating the BinaxNOW antigen test for SARS-CoV-2 by two groups of readers within 30 minutes resulted in high concordance (98.9%) in 2,110 encounters. BinaxNOW test sensitivity was 77.2% (258/334) compared to real-time reverse transcription-polymerase chain reaction. Same day antigen testing did not significantly improve test sensitivity while specificity remained high.

SARS-CoV-2 transmission in intercollegiate athletics not fully mitigated with daily antigen testing

Background

High frequency, rapid turnaround SARS-CoV-2 testing continues to be proposed as a way of efficiently identifying and mitigating transmission in congregate settings. However, two SARS-CoV-2 outbreaks occurred among intercollegiate university athletic programs during the fall 2020 semester despite mandatory directly observed daily antigen testing.

Methods

During the fall 2020 semester, athletes and staff in both programs were tested daily using Quidel's Sofia SARS Antigen Fluorescent Immunoassay (FIA), with positive antigen results requiring confirmatory testing with real-time reverse transcription polymerase chain reaction (RT-PCR). We used genomic sequencing to investigate transmission dynamics in these two outbreaks.

Results

In Outbreak 1, 32 confirmed cases occurred within a university athletics program after the index patient attended a meeting while infectious despite a negative antigen test on the day of the meeting. Among isolates sequenced from Outbreak 1, 24 (92%) of 26 were closely related, suggesting sustained transmission following an initial introduction event. In Outbreak 2, 12 confirmed cases occurred among athletes from two university programs that faced each other in an athletic competition despite receiving negative antigen test results on the day of the competition. Sequences from both teams were closely related and unique from strains circulating in the community, suggesting transmission during intercollegiate competition.

Conclusions

These findings suggest that antigen testing alone, even when mandated and directly observed, may not be sufficient as an intervention to prevent SARS-CoV-2 outbreaks in congregate settings, and highlights the importance of supplementing serial antigen testing with appropriate mitigation strategies to prevent SARS-CoV-2 outbreak in congregate settings.

Summary

High frequency, rapid turnaround SARS-CoV-2 testing continues to be proposed as a way of efficiently identifying and mitigating transmission in congregate settings. However, here we describe two SARS-CoV-2 outbreaks occurred among intercollegiate university athletic programs during the fall 2020 semester.

Influenza Vaccine Effectiveness Against Hospitalization in Fully and Partially Vaccinated Children in Israel: 2015-2016, 2016-2017, and 2017-2018

BACKGROUND

Influenza vaccine effectiveness (VE) varies by season, circulating influenza strain, age, and geographic location. There have been few studies of influenza VE among hospitalized children, particularly in Europe and the Middle East.

METHODS

We estimated VE against influenza hospitalization among children aged 6 months to 8 years at Clalit Health Services hospitals in Israel in the 2015-2016, 2016-2017, and 2017-2018 influenza seasons, using the test-negative design. Estimates were computed for full and partial vaccination.

RESULTS

We included 326 influenza-positive case patients and 2821 influenza-negative controls (140 case patients and 971 controls from 2015-2016, 36 case patients and 1069 controls from 2016-2017, and 150 case patients and 781 controls from 2017-2018). Over all seasons, VE was 53.9% for full vaccination (95% confidence interval [CI], 38.6%-68.3%), and 25.6% for partial vaccination (-3% to 47%). In 2015-2016, most viruses were influenza A(H1N1) and vaccine lineage-mismatched influenza B/Victoria; the VE for fully vaccinated children was statistically significant for influenza A (80.7%; 95% CI, 40.3%-96.1%) but not B (23.0%; -38.5% to 59.4%). During 2016-2017, influenza A(H3N2) predominated, and VE was (70.8%; 95% CI, 17.4%-92.4%). In 2017-2018, influenza A(H3N2), H1N1 and lineage-mismatched influenza B/Yamagata cocirculated; VE was statistically significant for influenza B (63.0%; 95% CI, 24.2%-83.7%) but not influenza A (46.3%; -7.2% to 75.3%).

CONCLUSIONS

Influenza vaccine was effective in preventing hospitalizations among fully vaccinated Israeli children over 3 influenza seasons, but not among partially vaccinated children. There was cross-lineage protection in a season where the vaccine contained B/Victoria and the circulating strain was B/Yamagata, but not in a season with the opposite vaccine-circulating strain distribution.

Patterns of Influenza Vaccination and Vaccine Effectiveness Among Young US Children Who Receive Outpatient Care for Acute Respiratory Tract Illness

IMPORTANCE

The burden of influenza among young children is high, and influenza vaccination is the primary strategy to prevent the virus and its complications. Less is known about differences in clinical protection following 1 vs 2 doses of initial influenza vaccination.

OBJECTIVES

To describe patterns of influenza vaccination among young children who receive outpatient care for acute respiratory tract illness in the US and compare vaccine effectiveness (VE) against medically attended laboratory-confirmed influenza by number of influenza vaccine doses received.

DESIGN

This test-negative case-control study was conducted in outpatient clinics, including emergency departments, at 5 sites of the US Influenza Vaccine Effectiveness Network during the 2014-2015 through 2017-2018 influenza seasons. The present study was performed from November 5, 2014, to April 12, 2018, during periods of local influenza circulation. Children aged 6 months to 8 years with an acute respiratory tract illness with cough who presented for outpatient care within 7 days of illness onset were included. All children were tested using real-time, reverse-transcriptase polymerase chain reaction for influenza for research purposes.

EXPOSURES

Vaccination in the enrollment season with either 1 or 2 doses of inactivated influenza vaccine as documented from electronic medical records, including state immunization information systems.

MAIN OUTCOMES AND MEASURES

Medically attended acute respiratory tract infection with real-time, reverse-transcriptase polymerase chain reaction testing for influenza.

RESULTS

Of 7533 children, 3480 children (46%) were girls, 4687 children (62%) were non-Hispanic white, and 4871 children (65%) were younger than 5 years. A total of 3912 children (52%) were unvaccinated in the enrollment season, 2924 children (39%) were fully vaccinated, and 697 children (9%) were partially vaccinated. Adjusted VE against any influenza was 51% (95% CI, 44%-57%) among fully vaccinated children and 41% (95% CI, 25%-54%) among partially vaccinated children. Among 1519 vaccine-naive children aged 6 months to 2 years, the VE of 2 doses in the enrollment season was 53% (95% CI, 28%-70%), and the VE of 1 dose was 23% (95% CI, -11% to 47%); those who received 2 doses were less likely to test positive for influenza compared with children who received only 1 dose (adjusted odds ratio, 0.57; 95% CI, 0.35-0.93).

CONCLUSIONS AND RELEVANCE

Consistent with US influenza vaccine policy, receipt of the recommended number of doses resulted in higher VE than partial vaccination in 4 influenza seasons. Efforts to improve 2-dose coverage for previously unvaccinated children may reduce the burden of influenza in this population.

Influenza Vaccine Effectiveness in the Inpatient Setting: Evaluation of Potential Bias in the Test-Negative Design by Use of Alternate Control Groups

The test-negative design is validated in outpatient, but not inpatient, studies of influenza vaccine effectiveness. The prevalence of chronic pulmonary disease among inpatients can lead to nonrepresentative controls. Test-negative design estimates are biased if vaccine administration is associated with incidence of noninfluenza viruses. We evaluated whether control group selection and effects of vaccination on noninfluenza viruses biased vaccine effectiveness in our study. Subjects were enrolled at the University of Michigan and Henry Ford hospitals during the 2014-2015 and 2015-2016 influenza seasons. Patients presenting with acute respiratory infection were enrolled and tested for respiratory viruses. Vaccine effectiveness was estimated using 3 control groups: negative for influenza, positive for other respiratory virus, and pan-negative individuals; it was also estimated for other common respiratory viruses. In 2014-2015, vaccine effectiveness was 41.1% (95% CI: 1.7, 64.7) using influenza-negative controls, 24.5% (95% CI: -42.6, 60.1) using controls positive for other virus, and 45.8% (95% CI: 5.7, 68.9) using pan-negative controls. In 2015-2016, vaccine effectiveness was 68.7% (95% CI: 44.6, 82.5) using influenza-negative controls, 63.1% (95% CI: 25.0, 82.2) using controls positive for other virus, and 71.1% (95% CI: 46.2, 84.8) using pan-negative controls. Vaccination did not alter odds of other respiratory viruses. Results support use of the test-negative design among inpatients.

Outcomes and Resource Use Among Overweight and Obese Children With Sepsis in the Pediatric Intensive Care Unit

OBJECTIVE

To evaluate the effect of overweight and obesity on outcomes and resource use among patients with sepsis in the pediatric intensive care unit (PICU).

DESIGN

Retrospective analysis of clinical characteristics, resource use, and mortality among children 0 to 20 years of age admitted to the C.S. MottChildren's Hospital PICU (University of Michigan) between January 2009 and December 2015, with a diagnostic code for sepsis at admission (based on International Classification of Diseases, Ninth Revision-Clinical Modification codes) and with weight and height measurements at PICU admission.

MEASUREMENTS AND MAIN RESULTS

A total of 454 participants met the inclusion criteria. Seventy-six were categorized as underweight (body mass index [BMI] percentile <5th) and were excluded, which left a final sample size of 378 participants. Children with a BMI >5th and <85th percentiles for age were categorized as normal weight and those with a BMI >85th percentile as overweight/obese. After descriptive and bivariate analyses, multivariate regression methods were used to assess the independent effect of obesity status on mortality and the use of PICU technology after adjustment for patient age and illness severity at admission. Of the 378 patients studied, 41.3% were overweight/obese. There was no difference in microbiologic etiology of sepsis (P = .36), median PICU length of stay in days (5.4 vs 5.6; P = .61), or PICU mortality (6.4% vs 7.2%; P = .76) by weight status. The use of specialized PICU technology including extracorporeal membrane oxygenation (odds ratio [OR]: 2.77, 95% confidence interval [CI]:1.13-6.79) and continuous renal replacement therapy (OR: 4.58, 95% CI: 1.16-18.0) was higher among overweight/obese patients, compared with normal weight patients.

CONCLUSIONS

Although PICU mortality and length of stay were similar for obese-overweight patients and normal weight critically ill children with sepsis, there was significantly higher use of specialized organ-supportive technology among obese patients, likely indicating higher occurrence of multiple organ dysfunction.

Influenza Vaccination and Treatment with Antiviral Agents Among Hospitalized Adults in the 2014–2015 and 2015–2016 Influenza Seasons

Background

Vaccination and treatment with neuraminidase inhibitors can reduce incidence and severity of influenza. Observational studies suggest antiviral treatment reduces influenza symptom duration and severe outcomes among hospitalized patients. The interaction of the effects of vaccination and antiviral treatment against severe influenza has not been established.

Methods

We used data from a test-negative influenza vaccine effectiveness study. The parent study enrolled adults admitted to two hospitals in Michigan with an acute respiratory illness of ≤10 days duration during the 2014–2015 and 2015–2016 influenza seasons. Respiratory swabs from enrolled patients were tested for influenza by RT-PCR; influenza-positive individuals were included in this analysis. We evaluated predictors of vaccination and antiviral treatment using logistic regression. We also assessed the association between antiviral treatment and hospital length of stay (LOS) using linear regression models stratified by vaccination status.

Results

We included 200 individuals in the analysis; 103 (51.5%) were vaccinated and 135 (67.5%) were treated with antivirals. Significant predictors of vaccination included age ≥65, white race, a Charlson comorbidity index (CCI) score ≥3, study site, and increased past-year health care visits. Antiviral treatment varied by study site and was more common in the 2015–2016 season and among those aged 18–49. Vaccination was not associated with antiviral treatment or with time from illness onset to treatment. Antiviral treatment was associated with reduced LOS (percent change in LOS: −23.6% (−39.2%, −4.1%), P = 0.02) among vaccinated participants but not among unvaccinated participants (21.1% (−10.9%, 64.8%), P = 0.22) after adjustment for sex, age, influenza subtype, site, CCI, frailty, and past-year health care contacts. When an interaction term was used in lieu of stratification the interaction was significant (P = 0.01). This difference in antiviral effectiveness by vaccination status held across age groups, but was most dramatic for those aged 18–49.

Conclusion

Vaccinated individuals were more likely than unvaccinated individuals to benefit from antiviral treatment. This finding warrants confirmation in other populations.

Disclosures

A. S. Monto, sanofi pasteur: Grant Investigator, Research grant. Novartis: Consultant, Consulting fee. Protein Sciences: Consultant, Consulting fee. E. T. Martin, Pfizer: Scientific Advisor, Research grant. Merck: Scientific Advisor, Research grant. Multiparty Group For Advice on Science: Scientific Advisor, Research grant.