Quantifying the rebound of influenza epidemics after the adjustment of zero-COVID policy in China

Abstract

The co-existence of coronavirus disease 2019 (COVID-19) and seasonal influenza epidemics has become a potential threat to human health, particularly in China in the oncoming season. However, with the relaxation of nonpharmaceutical interventions (NPIs) during the COVID-19 pandemic, the rebound extent of the influenza activities is still poorly understood. In this study, we constructed a susceptible-vaccinated-infectious-recovered-susceptible (SVIRS) model to simulate influenza transmission and calibrated it using influenza surveillance data from 2018 to 2022. We projected the influenza transmission over the next 3 years using the SVIRS model. We observed that, in epidemiological year 2021–2022, the reproduction numbers of influenza in southern and northern China were reduced by 64.0% and 34.5%, respectively, compared with those before the pandemic. The percentage of people susceptible to influenza virus increased by 138.6% and 57.3% in southern and northern China by October 1, 2022, respectively. After relaxing NPIs, the potential accumulation of susceptibility to influenza infection may lead to a large-scale influenza outbreak in the year 2022-2023, the scale of which may be affected by the intensity of the NPIs. And later relaxation of NPIs in the year 2023 would not lead to much larger rebound of influenza activities in the year 2023-2024. To control the influenza epidemic to the pre-pandemic level after relaxing NPIs, the influenza vaccination rates in southern and northern China should increase to 53.8% and 33.8%, respectively. Vaccination for influenza should be advocated to reduce the potential reemergence of the influenza epidemic in the next few years.

Challenges and Strategies for Influenza Response After COVID-19 Pandemic

“…In epidemiological year 2021/2022, the reproduction number of influenza was reduced by 64.0% (in southern China) and 34.5% (in northern China), compared with those before the COVID-19 pandemic. [23] This scenario was also observed globally, although COVID-19 containment measurements varied in different regions, such as the United States, which experienced a rebound of respiratory diseases in the first winter after relaxing pandemic prevention and control measures. This demonstrates that NPIs can effectively reduce the transmission of respiratory infections at the population level, thereby reducing the burden of disease in the real world.…”

“…Thus, advocating influenza vaccination is crucial for mitigating potential influenza epidemics in the coming years. [23] The world has won the battle against the COVID-19 pandemic by implementing NPIs and massive population vaccinations. The WHO declared the end of the pandemic on May 5, 2023.…”

“…One study showed that the proportion of individuals susceptible to the influenza virus increased by 138.6% and 57.3% in southern and northern China, respectively, by October 1, 2022. [23] Environmental factors may also contribute to the rebound in influenza activity. A previous study suggested that temperature is significantly correlated with influenza activity in China.…”

Increased population susceptibility to seasonal influenza during the COVID‐19 pandemic in China and the United States

“…With the lifting of PHSMs, there is potential for the emergence of influenza epidemics, which were initially identified and estimated in studies during the pandemic. 4,5 However, after the end of the COVID-19 emergency, 6 people in the United States, which declared coexistence with COVID-19 in March 2022, 7 and China, which began easing PHSMs in December 2022, 8 largely returned to their pre-pandemic social life.…”

“…There are no uniform criteria for assessing influenza susceptibility. Measures from an epidemiological perspective include vaccine effectiveness, 12 the proportion of susceptible individuals, 5 and measures from a systems biology perspective include genetic testing and immune-level measurements from a systems biology perspective. 13 For a given country, the external social living conditions, such as population movement, climate, and temperature, are essentially the same after the lifting of COVID-19 PHSMs as before the beginning of the COVID-19 pandemic.…”