Health professionals have expressed mixed opinions on the guidance. Some were concerned that reducing the isolation period could be accelerating the rapid spread of the Omicron variant. According to research cited by the CDC, COVID-19 infectiousness generally peaks the day before symptom onset. In addition, a negative antigen test may indicate that the period of infectiousness has ended.
When Will Symptoms Appear?
Compared to previous variants, Omicron reportedly has a shorter incubation period—around two to four days—meaning symptoms will develop faster after exposure.
In one study of an Omicron outbreak in Oslo, Norway, following a Christmas party in November, researchers found that symptoms began in 45% of the 80 confirmed or probable cases three days after the party. In a study of a similar outbreak in South Korea, the median observed incubation period was slightly longer, at 4.2 days.
But symptom onset doesn’t always coincide with infectiousness. Over 20% of transmission can be attributed to individuals who are asymptomatic or presymptomatic (one to two days before symptom onset).
When Is Someone Most Contagious During a COVID-19 Infection?
The CDC cited a 2020 review of 113 studies done in 17 countries with evidence that COVID-19 viral load peaks around the time of symptom onset. Reviewers concluded that the full period of contagiousness ranges from two to three days before and eight days after symptom onset.
This means people infected with COVID-19 are most contagious when they start experiencing symptoms, with the potential for transmission declining rapidly after that and disappearing after eight days.
Jin Su Song, MD, MPH, DTMH, an infectious disease specialist working at Korea Disease Control and Prevention Agency, said his team observed Omicron contagiousness disappearing completely after 10 days. The current isolation guidance in South Korea is 10 days.
“We can’t exactly know how long [infection] lasts,” Song told Verywell. “According to our research, and also our preliminary studies show that the duration of infection is probably equal or less than the wild-type virus or Delta virus.”
When Should You Test?
If you were exposed to someone who tested positive for COVID-19, it’s wise to get tested two to four days after the exposure, given Omicron’s incubation period.
PCR tests have a lower threshold that can detect the virus before the period of infectiousness, making them more accurate for those who have been infected even if they were asymptomatic.
Daniel Larremore, PhD, an assistant professor at the University of Colorado, Boulder, said that while PCR tests remain the “gold standard” for clinical diagnosis, the delay may be a tradeoff that isn’t worth the increased accuracy for a large-scale testing program.
“With high demand, PCR tests are often delayed by a day or more, meaning that even though the PCR is more sensitive to detect early infections, that information comes back at a considerable delay, essentially wiping out most of the ‘early warning’ value that one might hope for,” he wrote in an email to Verywell.
If you take a PCR test, isolating before getting the results is important to prevent potential spread to others.
The high-sensitivity of PCR tests means they may also show up positive in the post-infectious period, after one has recovered and is no longer contagious. So a negative antigen test and a positive PCR could mean you are about to be contagious, or are no longer contagious. In that case, the timing of exposure can help determine which phase a COVID-positive person is in.
The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.
