Estimating the impact of low influenza activity in 2020 on population immunity and future influenza seasons in the United States

Backgrounds Influenza activity in the 2020-21 season was remarkably low, likely due to implementation of public health preventive measures such as social distancing, mask-wearing, and school closure. With waning immunity, the impact of low influenza activity in the 2020-21 on the following season is unknown. Method We built a multi-strain compartmental model that captures immunity over multiple influenza seasons in the US. Compared to the counterfactual case, where influenza activity remained at the normal level in 2020-21, we estimated the change in the number of hospitalizations when the transmission rate was decreased by 20% in 2020-21. We varied the level of vaccine uptake and effectiveness in 2021-22. We measured the change in population immunity over time by varying the number of seasons with lowered influenza activity. Results With the lowered influenza activity in 2020-21, the model estimated 102,000 [95% CI: 57,000-152,000] additional hospitalizations in 2021-22, without changes in vaccine uptake and effectiveness. The estimated changes in hospitalizations varied depending on the level of vaccine uptake and effectiveness in the following year. Achieving 50% increase in vaccine coverage was necessary to avert the expected increase in hospitalization in the next influenza season. If the low influenza activity were to continue over several seasons, population immunity would remain low during those seasons, with 48% the population susceptible to influenza infection. Conclusion Our study projecteda large compensatory influenza season in 2021-22 due to a light season in 2020-21. However, higher influenza vaccine uptake would reduce this projected increase in influenza.

The COVID-19 Pandemic and the Incidence of the Non-COVID-19 Pneumonia in Adults

Introduction: The coronavirus disease 2019 (COVID-19) lockdown strategies were associated with a significant decrease in the common respiratory viral diseases and decreased the need for hospitalization among children in the COVID-19 outbreak. However, the trend of non-COVID-19 pneumonia in adult people remains uncertain. Our aim is to assess the impact of the COVID-19 pandemic on the incidence of the non-COVID-19 pneumonia in adult people and understand whether the substantial decrease in pneumonia cases is the same as the decline in the incidence of respiratory viral disease activity.Methods: We conducted a retrospective analysis of adult patients presenting with pneumonia from January 2019 to December 2020. Details on all the demographics of the patient of pneumonia, hospital course details, prior admission history within 3 months, respiratory culture, and antibiotics sensitivity test were also obtained.Results: The number of adult patients with community-acquired pneumonia in 2020 was lower than that in 2019, which decreased by 74 patients in 2020. The decreasing number of patients with community-acquired pneumonia between 2019 and 2020 was from −13.9% in January to March 2020 to −39.7% in October to December 2020. The decreasing number of patients with community-acquired pneumonia between 2019 and 2020 was from −14.8% in the youngest cohort to −28.7% in those aged ≥85 years. The number of reduced patients with community-acquired pneumonia is greater in late seasons and older age, respectively. The number of adult patients with hospital-acquired pneumonia in 2020 was lower than that in 2019, which decreased by 23 patients in 2020. The decreasing number of patients with hospital-acquired pneumonia between 2019 and 2020 was from −20.0% in January to March 2020 to −52.4% in October to December 2020. The decreasing number of patients with hospital-acquired pneumonia between 2019 and 2020 was from 0% in the youngest cohort to −45.6% in those aged ≥ 85 years. The number of reduced patients with hospital-acquired pneumonia is greater in late seasons and older age, respectively.Conclusion: Interventions applied to control the COVID-19 pandemic were effective not only in substantial changes in the seasonal influenza activity, but also in decreasing adult pneumonia cases.

96. Relative Vaccine Effectiveness Against Influenza-Related and Any Respiratory-Related Hospital Encounter During the 2019/20 High Influenza Activity Period: A Comprehensive Real-World Analysis to Compare Quadrivalent Cell-based and Egg-based Influenza Vaccines

Background Changes in the influenza hemagglutinin protein during replication of influenza in eggs during vaccine production may contribute to low vaccine effectiveness (VE). This phenomenon, egg adaptation, can explain VE differences between egg-based (QIVe-SD) and cell-based (QIVc) quadrivalent influenza vaccines. This research evaluated the relative vaccine effectiveness (rVE) of QIVc versus QIVe-SD in the reduction of influenza-related and any respiratory-related hospitalizations/emergency room (ER) visits among subjects 4-64 years old during the 2019/20 influenza season. Methods A retrospective cohort analysis was conducted among subjects 4-64 years old vaccinated with QIVc or QIVe-SD using administrative claims data in the U.S. (IQVIA PharMetrics® Plus). The adjusted number of events and rates of influenza-related hospitalizations/ER visits and respiratory-related hospitalizations/ER visits were assessed using inverse probability of treatment weighting (IPTW). Poisson regression was used to estimate relative vaccine effectiveness (rVE). In the main analysis, the study period was from Aug 4, 2019 to Mar 7, 2020 (ending early to avoid any influenza outcome misclassification with COVD-19 infection). In the assessment of the high influenza activity period (HIAP), the analysis period was restricted to Dec 8, 2019 to Mar 7, 2020. Results During the 2019/20 influenza season, 1,150,134 recipients of QIVc and 3,924,819, of QIVe-SD were identified following IPTW. In the main analysis, adjusted results show that QIVc was associated with a significantly higher rVE compared to QIVe-SD against influenza-related hospitalizations/ER visits (5.3% [95%CI: 0.5%-9.9%]) and respiratory-related hospitalizations/ER visits (8.2% [95%CI: 6.5%-9.8%]). Similarly, in the HIAP analysis, QIVc was associated with a significantly higher rVE compared to QIVe-SD for influenza-related hospitalizations/ER visits (5.7% [95%CI: 0.8%-10.4%]) and respiratory-related hospitalizations/ER visits (7.3% [95%CI: 5.4%-9.2%]). Conclusion QIVc was more effective in preventing influenza-related and respiratory-related hospitalizations/ER visits compared to QIVe-SD, using either a broad influenza season definition or restricting to the HIAP. Disclosures Victoria Divino, PhD, Seqirus (Consultant) Maarten Postma, Dr., Seqirus (Consultant) Stephen I. Pelton, MD, Seqirus (Consultant) Joaquin F. Mould-Quevedo, PhD, Seqirus (Employee) Ruthwik Anupindi, PhD, Seqirus (Consultant) Mitchell DeKoven, PhD, Seqirus (Consultant) myron J. levin, MD, GSK group of companies (Employee, Research Grant or Support)

1444. Influenza Surveillance in the Veterans Health Administration (VHA): 2020-2021 Season

Background Veterans Health Administration’s (VHA) large elderly population is at higher risk for influenza complications, including hospitalization and death. Herein we summarize VHA’s national annual surveillance data for seasonal influenza activity and vaccinations. Methods Influenza telephone triage, influenza-like-illness (ILI) encounters and antiviral prescriptions plus outpatient visits, laboratory testing, hospitalizations and deaths for influenza were obtained from VHA data sources (9/27/20-5/22/21) and compared to prior years and CDC FluView data. Influenza vaccinations were captured from 8/1/2020. Vaccination rates were calculated based on VHA users during the fiscal year. Results Surveillance metrics are presented (Table). Vaccinations were decreased and ILI was below average (0.3%-0.7% per week). Activity was highest 2020 Weeks 46-47 but remained low the entire season with no distinct peak seen, matching national influenza activity (Fig. 1). Testing revealed 161 influenza positives from 440,553 tests performed (0.04%). Hospitalizations among laboratory-confirmed cases were similar to the prior season (16% vs 17%). Median length of stay (6 days) and deaths (17, 12%) were increased over prior seasons. Among 15 deaths where results were available, 4 had Influenza A, 10 had Influenza B and 1 had Influenza A+B. Nine were co-infected with COVID-19. Total influenza positives, outpatient visits, hospitalizations and antiviral use were extremely low compared to all prior season where national VHA data has been analyzed (Table, Fig. 2). Conclusion Overall, influenza vaccination levels were decreased although percent receiving high-dose formulation was stable. Despite lower vaccination rates, the 2020-21 influenza season was of historically low activity, even with markedly increased testing performed in the setting of multiplex tests for influenza with COVID-19. Deaths were primarily seen with Influenza B and among those co-infected with COVID-19. This may also have contributed to increased length of stay. VHA influenza activity continues to track closely with national CDC data and may have been impacted by mitigation measures used to contain COVID-19, which were likely effective in curbing influenza activity. Disclosures All Authors: No reported disclosures

National Influenza Annual Report, Canada, 2020–2021, in the global context

During the 2020–2021 Canadian influenza season, no community circulation of influenza occurred. Only 69 positive detections of influenza were reported, and influenza percent positivity did not exceed 0.1%. Influenza indicators were at historical lows compared with the previous six seasons, with no laboratory-confirmed influenza outbreaks or severe outcomes being reported by any of the provinces and territories. Globally, influenza circulation was at historically low levels in both the Northern and the Southern Hemispheres. The decreased influenza activity seen in Canada and globally is concurrent with the implementation of non-pharmaceutical public health measures to mitigate the spread of the coronavirus disease 2019 (COVID-19). Although it is difficult to predict when influenza will begin to re-circulate, given the increased COVID-19 vaccination and the relaxation of public health measures, an influenza resurgence can be expected and may be more severe or intense than recent seasons. Influenza vaccination, along with non-pharmaceutical public health measures, continues to remain the best method to prevent the spread and impact of influenza. Public health authorities need to remain vigilant, maintain surveillance and continue to plan for heightened seasonal influenza circulation.

Spotlight influenza: The 2019/20 influenza season and the impact of COVID-19 on influenza surveillance in the WHO European Region

Background Annual seasonal influenza activity in the northern hemisphere causes a high burden of disease during the winter months, peaking in the first weeks of the year. Aim We describe the 2019/20 influenza season and the impact of the COVID-19 pandemic on sentinel surveillance in the World Health Organization (WHO) European Region. Methods We analysed weekly epidemiological and virological influenza data from sentinel primary care and hospital sources reported by countries, territories and areas (hereafter countries) in the European Region. Results We observed co-circulation of influenza B/Victoria-lineage, A(H1)pdm09 and A(H3) viruses during the 2019/20 season, with different dominance patterns observed across the Region. A higher proportion of patients with influenza A virus infection than type B were observed. The influenza activity started in week 47/2019, and influenza positivity rate was ≥ 50% for 2 weeks (05–06/2020) rather than 5–8 weeks in the previous five seasons. In many countries a rapid reduction in sentinel reports and the highest influenza activity was observed in weeks 09–13/2020. Reporting was reduced from week 14/2020 across the Region coincident with the onset of widespread circulation of SARS-CoV-2. Conclusions Overall, influenza type A viruses dominated; however, there were varying patterns across the Region, with dominance of B/Victoria-lineage viruses in a few countries. The COVID-19 pandemic contributed to an earlier end of the influenza season and reduced influenza virus circulation probably owing to restricted healthcare access and public health measures.