Study: Incubation Period of COVID-19 Strains Decrease Over Time

The Delta and Omicron COVID-19 variants, which are the dominant strains in many countries around the world, have a significantly shorter incubation period than other variants, according to a study published in JAMA Open Network. The World Health Organization (WHO) continues to recommend isolating for 14 days after contracting any SARS-CoV-2 strain.

“SARS-CoV-2 has evolved and mutated continuously throughout the COVID-19 pandemic, producing variants with different enhanced transmission and virulence,” the study authors wrote. “Identifying the incubation period of different variants is a key factor in determining the isolation period.”

Understanding the incubation period of a disease is important to identify cases, manage emerging threats, perform contact tracing, determine secondary cases, and establish public health programs to reduce further transmission. For many diseases, the duration of incubation period can also help determine isolation length for infected individuals.

The COVID-19 variants are broken into variants of interest (VOI) and variants of concern (VOC), with the latter being highly transmissible, resistant to vaccines, and sometimes elusive to diagnostic testing. SARS-CoV-2 VOCs are Alpha (B.1.1.7), Beta (B.1.351), Gamma (P.1), Delta (B.1.617.2), and Omicron (B.1.1.529).

Using a meta-analysis, researchers pooled data of the overall incubation period of COVID-19. They also pooled the incubation periods of each variant, with the aim of adjusting prevention and control strategies to decrease COVID-19 and its transmission.

The team looked at 8112 patients across 142 studies, primarily conducted in China between January and March 2020. Investigators found that the pooled incubation period for all variants, including the Alpha, Beta, Delta, and Omicron, was 6.57 days, though it ranged from 1.80 to 18.87 days. Separated analyses of the individual variants were also reported.

The Alpha COVID-19 strain had a mean incubation period of 5 days. Pooled data for the Beta variant calculated the incubation period to be 4.5 days, with the Beta/Gamma lasting 5.10 days and the incubation periods of Delta and Omicron strains lasting 4.41 days and 3.42 days, respectively.

Among different subgroups, 8 studies reported a longer incubation period among patients older than 60 years of age. Strains had a mean incubation period of 7.43 days, which was longer than the 6.65 days for the general population, but not significantly. The researchers conceded that older patients were less likely to report symptoms early on, waiting to report on them until it was severe.

Children 18 years of age and younger experienced a mean incubation period of 8.82 days for all COVID-19 strains, significantly greater than the general population. Additionally, those with no severe illness had a longer incubation period of 6.99 days.

The team addressed limitations to the study, the first being that researchers retrospectively collected data, which results in recall bias. They suggested that confounding variables could impact the results, including population factors and heterogenous data. Additionally, the resulting incubation period was determined during a short study time, which could generalize findings.

The team concluded that “the results may be helpful in changing public health guidance on duration of quarantine, outbreak investigation, and contact tracing.”

Reference

Wu, Yu. Kang, Liangyu, Guo, Zirui. Incubation Period of COVID-19 Caused by Unique SARS-CoV-2 Strains. JAMA Netw Open. 2022;5(8):e2228008. doi:10.1001/jamanetworko