A first step towards pandemic-proof outbreak detection in the Netherlands

Abstract Background Reporting of notifiable infectious diseases was overall impacted by the COVID-19 pandemic. This could affect disease surveillance and thus outbreak detection. In this study, we take the first steps in the development of a methodology that adjusts for the impact of the COVID-19 pandemic on the number of notification of notifiable infectious diseases and provides corrected alarm thresholds for outbreak detection. Methods We analyzed cases of 25 notifiable infectious diseases reported from 2015 - 2023 in the Netherlands. Negative binomial regression was used to calculate the incidence rate ratios for each period: pre-COVID, COVID 2020, COVID 2021, COVID 2022 and post-COVID. To address the decrease in notifications during COVID, we tested three correction methods 1) recoding COVID years as missing; 2) imputing with the last pre-COVID observation; and 3) imputing the historical moving average. Results Malaria, typhoid fever, hepatitis A, meningococcal infection, paratyphoid fever, Q-fever, shigellosis, measles, mumps and pertussis had significantly lower notifications during the COVID-19 pandemic, but the duration and magnitude of the effect differed among the infections. Additionally, the newly calculated alarm thresholds showed a noticeable difference compared to the original unadjusted alarm thresholds. However, the variation among the three different corrected alarm thresholds was not substantial. Conclusions During the COVID-19 pandemic, notifications of ten infectious diseases declined significantly. The duration of this decline varied among infections, highlighting the need for disease-specific adjustments. Our study demonstrates that accounting for the reduced notifications impacts alarm threshold calculations for outbreak detection. Further validation by communicable disease control professionals is essential to assess the applicability of the adjusted alarm thresholds for outbreak detection. Key messages • The COVID-19 pandemic led to declines in infectious disease notifications, which may hinder early outbreak detection. • Our study highlights the need to recalibrate outbreak signals after COVID-19 to support accurate public health action.