P21 Social Distancing and Trends in Influenza Hospitalization during the COVID-19 Outbreak: A Difference-in-Difference Analysis of German Claims Data


      As COVID-19 spread worldwide, indicators of influenza activity in the Northern Hemisphere began to decline by mid-to-late February. In Germany, federal lockdown measures were introduced to contain the outbreak on 22/03/2020 (week 12). We used claims data from AOK PLUS, a regional sickness fund covering around half the population in Saxony and Thuringia (6.2 million inhabitants), to examine the trend of influenza hospitalizations in 2020 compared to 2019.


      Using data from 01/01/2019 to 31/05/2020 (weeks 1-22), influenza hospitalizations were identified using ICD-10-GM codes J10-J11. We estimated changes in the number of influenza hospitalizations using a “difference-in-differences” model including variables for age group (<18, 18-44, 45-64, 65-79, 80+), gender, week, year, and outbreak status (interaction variable between year 2020 and week 12 or later). Adjusted incidence rate ratios (aIRRs) were estimated using Poisson regression with heteroskedasticity-robust standard errors.


      During weeks 1-22, we observed 5,174 influenza hospitalizations in 2019 and 2020. Influenza hospitalizations in 2020 showed similar trends until week 12 and then showed a relative decline compared to 2019. The average number of influenza hospitalizations per week during weeks 12-22 significantly decreased in 2020 compared to 2019 (1.6 vs. 5.2; aIRR: 0.45; 95% CI: 0.34-0.59; p<0.001). When stratified by age group, all groups except age 18-44 had a similar decrease in average influenza hospitalizations per week in 2020 compared to 2019, with large relative declines in patients age 80+ (2.2 vs. 5.8; aIRR: 0.36; 95% CI: 0.28-0.46; p<0.001) and children <18 (1.8 vs. 8.0; aIRR: 0.38; 95% CI 0.32-0.46; p<0.001).


      The number of influenza hospitalizations saw a relative faster decline in 2020 compared to 2019 after the introduction of federal lockdown measures in Germany, possibly due to the effectiveness of non-pharmaceutical interventions like social distancing and the use of facemasks.