Among Covid-19’s pernicious features, its incubation period is longer than many other respiratory viral infections, including influenza, respiratory syncytial virus and rhinovirus. The good news is the interval between exposure and the development of symptoms appears to be narrowing.

Scientists from Peking University and Tsinghua University in Beijing analyzed data from more than 140 studies to estimate the incubation period of Covid caused by different strains of SARS-CoV-2. It fell from an average of five days with an alpha infection to 3.42 days with omicron, according to a study published Monday in the journal JAMA Network Open.

The researchers also noted variable incubation periods across different age groups and severity of disease.

“The findings of this study suggest that SARS-CoV-2 has evolved and mutated continuously throughout the Covid-19 pandemic, producing variants with different enhanced transmission and virulence,” Wannian Liang and colleagues said. “Identifying the incubation period of different variants is a key factor in determining the isolation period.”

The findings are significant for places like China and Hong Kong, which maintain a Covid Zero policy intended to eliminate any signs of the virus as quickly as possible. Everyone who is infected and all foreign travelers are expected to isolate throughout the potential incubation period to prevent transmission to others. Both China and Hong Kong have recently reduced quarantine periods for new arrivals as part of their efforts to revive their economies.