The average incubation period for SARS-CoV-2 has decreased as the virus evolved, from 5 days for the Alpha variant to 3 days for the currently predominant Omicron variant, suggests a recent study published in JAMA Network Open.¹

To determine the incubation periods of Covid-19 caused by different SARS-CoV-2 variants, researchers from China conducted a systematic review and meta-analysis of 142 studies published between December 2019 and February 2022. Of the studies included, 45 were rated as providing strong evidence, 82 as moderate evidence and 15 were assessed as weak. These studies involved a total of 8112 patients.

The overall incubation period was found to be 6.57 days and ranged from 1.80 to 18.87 days. Infection due to the Alpha (B.1.1.7) variant resulted in a mean incubation period of 5 days. For Beta variant (B.1.351), the incubation period was 4.50 days and for the Delta variant (B.1.617.2), it was 4.41 days and for the Omicron variant (B.1.1.529), the incubation period was 3.42 days.

The incubation periods for different age groups and according to severity of infection were also analysed. Among patients aged 60 years and older, the mean incubation period was 7.43 days, whereas it was 8.82 days among patients aged 18 years and younger. Among patients with severe infection, the incubation period was around 6.7 days versus ~7 days in patients with mild or moderate illness.

The study also found that compared to the other acute respiratory viral illnesses, Covid-19 had a longer incubation period, which was 1.4 days for rhinovirus, 1.43-1.64 days for influenza A, 2.6 days for parainfluenza, 32 days for human coronavirus, 4 days for severe acute respiratory syndrome (SARS) and 4.4 days for respiratory syncytial virus (RSV).

Since it was first identified in 2019, the SARS-CoV-2 has undergone several mutations from the ancestral strain into different variants of concern and variants of interest, which differed in virulence and infectiousness. This systematic review and meta-analysis has for the first time compared the incubation periods of the different SARS-CoV-2 variants. From the Alpha variant to the Omicron strain, the incubation period has declined with Omicron having the shortest incubation period. The authors concluded by noting that “identifying the incubation period of different variants is a key factor in determining the isolation period”. These study findings may impact public health advisories on quarantine and isolation and reduce the burden on hospitals.

Reference

1. Yu Wu, et al. Incubation period of COVID-19 caused by unique SARS-CoV-2 strains: a systematic review and meta-analysis. JAMA Netw Open. 2022 Aug 1;5(8):e2228008. doi: 10.1001/jamanetworkopen.2022.28008.