Asymptomatic SARS-CoV-2 infection and the demography of COVID-19

Wang, Z.; Wu, P.; Wang, J.; Lourenco, J.; Li, B.; Rader, B.; Laine, M.; Miao, H.; Wang, L.; Song, H.; Bharti, N.; Brownstein, J.; Bjornstad, O. N.; Dye, C.; Tian, H.

Asymptomatic individuals carrying SARS CoV 2 can transmit the virus and contribute to outbreaks of COVID 19, but it is not yet clear how the proportion of asymptomatic infections varies by age and geographic location. Here we use detailed surveillance data gathered during COVID 19 resurgences in six cities of China at the beginning of 2021 to investigate this question. Data were collected by multiple rounds of city wide PCR test with detailed contact tracing, where each patient was monitored for symptoms through the whole course of infection. We find that the proportion of asymptomatic infections declines with age (coefficient =-0.006, P<0.01), falling from 56% in age group 0~9 years to 12% in age group >60 years. Using an age-stratified compartment model, we show that this age-dependent asymptomatic pattern together with the age distribution of overall cases can explain most of the geographic differences in reported asymptomatic proportions. Combined with demography and contact matrices from other countries worldwide, we estimate that a maximum of 22%~55% of SARS CoV 2 infections would come from asymptomatic cases in an uncontrolled epidemic based on asymptomatic proportions in China. Our analysis suggests that flare ups of COVID 19 are likely if only adults are vaccinated and that surveillance and possibly control measures among children will be still needed in the future to contain epidemic resurgence.