COVID-19 Is at Peak Infectiousness for 5 Days After Symptom Onset

A study published in The Lancet Respiratory Medicine found that up to two-thirds of patients with COVID-19 could transmit the virus 5 days after the onset of symptoms, and one-fourth of patients could transmit the virus after 7 days. They also found that infectiousness lasts a median of 5 days after symptoms began. This information could help inform how to reduce spread of the virus through informed self-isolation.

The researchers used data from the Assessment of Transmission and Contagiousness of COVID-19 in Contacts (ATACCC) study conducted in the United Kingdom. The study had 2 enrollment periods: ATACCC1, which enrolled patients from September 13, 2020, to March 31, 2021, and ATACC2, which enrolled patients from May 24, 2021, to October 28, 2021. Patients were deemed fully vaccinated if they had their second COVID-19 vaccination 14 days or more before the index symptom onset, as none of the participants had their boosters.

All patients completed a daily symptom diary to track symptom onset. The primary outcome of the study was to define the window of COVID-19 infectiousness from the onset of infection and its correlation with symptom onset.

There were 393 participants enrolled in ATACCC1 and 345 participants in ATACCC2, of which 57 patients with cases of COVID-19 were included in the final study population. Participants were mostly White (89%) and had a median (IQR) age of 41 (29-49) years.

There were 25 cases (44%) in fully vaccinated individuals, all of whom had the Delta variant of SARS-CoV-2, and the 32 unvaccinated cases had the pre-Alpha (41%), Alpha (38%), and Delta (22%) variants. There were 53 cases (93%) that shed viral RNA for more than 7 days, and 51 (93%) had viral shedding that could be detected as plaque-forming units, captured in 49 cases. The researchers found, by characterizing the window of infectiousness in 42 cases, that shedding of infectious virus could occur for a median (IQR) of 5 (3-7) days.

Most cases (63%) had a peak infectious viral load on the same day that there was a peak in the RNA viral load, with only 5 (10%) having a peak infectious viral load before their peak RNA viral load. Only 9 of 38 cases (25%) with a definitive symptom onset date shed infectious virus before their symptoms began. Peak RNA viral load and peak infectious viral load were a median of 3 days after symptom onset.

Plaque assays were used to assess the probability of infectious virus presence. There were 25 (74%) of 34 cases that remained infectious for 5 days after the first PCR results were positive and 11 (32%) that remained infectious after 7 days. Similar rates were observed when using symptom onset as the first day, with 22 (64%) of cases still infectious after 5 days and 8 (24%) cases still infectious after 7 days.

There were some limitations to this study. Young and older patients were not represented well in this study. Upper respiratory tract swabs to collect samples were self-performed by participants, which may result in variability in sensitivity and specificity. The study was conducted before the emergence of the Omicron variant, which prevented evaluation of its RNA and infectious viral loads. The study’s findings should also be interpreted as relating to potential infectiousness, not transmission, as infectious virus presence does not inevitably lead to transmission.

The researchers concluded that they had found a window in which infectiousness was at its highest and the virus was most likely to transmit. They noted that guidance simply allowing individuals to end isolation once 5 days pass since symptom onset would result in two-thirds of individuals with infectious potential reentering the community.

“Preliminary evidence from our study has already informed policy and the real-world evidence presented here could be used to improve infection control policies and optimize guidance on self-isolation to minimize secondary transmission,” the authors wrote.

Reference

Hakki S, Zhou J, Jonnerby J, et al. Onset and window of SARS-CoV-2 infectiousness and temporal correlation with symptom onset: a prospective, longitudinal, community cohort study. Lancet Respir Med. Published online August 18, 2022. doi:10.1016/s2213-2600(22)00226-0