Emily Cai

New research from scientists at the Yale and Harvard Schools of Public Health suggests that the commonly-accepted five-day isolation period for those who test positive for COVID-19 may end when people are still infectious.

The study, which is currently prepublished — meaning it has not yet been peer-reviewed — investigates the time period of viral proliferation and clearance for the SARS-CoV-2 Omicron variant. Researchers analyzed PCR COVID-19 tests of 537 individuals and found that many samples remained positive for between five and 10 days following the initial positivity.

“This study demonstrates that the current 5 day isolation period may well be too short for a significant number of infected individuals,” Howard Forman, professor of economics, management and public health policy wrote in an email to the News. “If our intention is to stamp outspread, we should reconsider a time-based process to exit isolation and reconsider using antigen testing to test out.”

The Omicron COVID-19 variant is highly transmissible and infects a higher proportion of individuals who are vaccinated or have prior immunity than other variants, research conducted on the variant over the last several months has shown. These traits of the variant make it necessary to research how long the virus takes to clear up and how often it proliferates in individuals, according to Nathan Grubaugh, an associate professor of epidemiology.

“Over the past two years, we have been collecting data on the PCR values from very dense samplings. … Our goal was to use the data we had collected to determine what is the potential fraction of individuals in our sampling that still might be infectious on day five, day six, or day seven so we might better inform our isolation protocols,” Grubaugh, a senior author of the paper, said in an interview with the News.

Through its occupational health program, the National Basketball Association provided the research team with PCR test samples from 537 individuals for analysis. The testing program periodically tested symptomatic and asymptomatic individuals, and tested positive individuals daily in order to obtain this dense sampling.

Through this analysis, the researchers were able to measure viral RNA, or the genetic material of viruses. An analysis of the viral RNA allowed for a deeper understanding of the extent of viral shedding of the Omicron variant. Viral shedding is the rate and intensity by which the virus reproduces inside host cells. Rates of viral shedding help inform policies on isolation for individuals who test positive, as it describes the general period of time that an individual could be infectious.

“These findings and others indicate that there is much variability in the period that people can infect others,” Harlan Krumholz ’80, professor of medicine and public health policy, wrote in an email to the News. “Some people may clear the virus rapidly and others continue to spread it to others for 10 or more days. Setting a time amount and applying it to everyone will promote the spreading of the virus.”

Through the PCR tests, the researchers analyzed the Ct values, or cycle threshold values, of individuals. Ct values represent how many cycles of DNA amplification it takes for the COVID-19 genetic material to be detected within the sample. Ct values less than 30 usually correspond to a positive COVID-19 rapid antigen test, while values above 30 indicate non-infectiousness for COVID-19.

The study showed that most individuals tested reached high viral RNA after initially testing positive, with many individuals having Ct values less than 30 even five days after the initial positive test result. This meant that those individuals would still be testing positive on the rapid antigen test five days after their initial positive result. Per the study’s findings, positivity does not persist beyond day 10, as all individuals in the study had Ct values above 30 by day 11.

Grubaugh’s lab and colleagues aim to replicate the study in a larger, more representative population in order to best model the viral shedding and infectiousness, leading to better creation of isolation policies. Currently, the CDC recommends that positive individuals quarantine for five days if they are asymptomatic and then strictly wear a mask around others for five days after. 

Yale requires that students who test positive quarantine in isolation housing, and, if by day five they have waning or no symptoms, they may test and be released from quarantine if they test negative.