Variant watch: Ending isolation after Omicron

If you have had Covid-19 in this wave, which is almost entirely driven by the Omicron variant, what is the best time for you to end your isolation?

Some confusion has persisted around this, particularly since the US Centers for Disease Control (US CDC) recommended people end their isolation five days after they have a positive test.

Now, a study from Japan, albeit with a small sample size involving 21 people, suggests people can in some cases remain infectious up to 9 days after their symptoms begin.

The study was carried out by scientists from the National Institute of Infectious Disease (NIID), which measured the amount of viral RNA detected among the 21 cases, as well as for how long they could culture the virus from the samples of these people.

“The positive virus isolation results showed a similar trend as the viral RNA amount, and no infectious virus in the respiratory samples was detected after 10 days since diagnosis or symptom onset,” they said, implying 10 days of isolation was sufficient.

The data they released showed that the highest amounts of virus was cultured between days 3 and 6. If the virus can be cultured, it means the sample could lead to an infection, instead of merely consisting of viral RNA, which can also consist of dead Sars-CoV-2 remnants.

Experts believe this provides evidence that the US CDC advisory may be premature.

Different countries have followed distinct approaches to this question. In India, the government has advised that people should end their isolation on day 8 if they have not had any symptoms between days 5-7.

The UK too has a seven-day window, but encourages people to take two antigen tests, spaced 24 hours apart, on the last two days. If both are negative, you are safe to go out.

Antigen tests have long been regarded as a crude correlate of infectivity, even though they are substantially less accurate than RT-PCR tests. How they should be used needs careful consideration, particularly in the case of Omicron infections.

A new study, by researchers from University of Washington, found that antigen tests detected an infection three days after RT-PCR tests did in the case of Omicron infections, which meant that people could have been infectious before a home test would catch it.

The ideal then is likely a mix of the Indian and the UK guidance. Two tests towards the end of an infection period, taken at least after the sixth day, is likely to reflect true infectiousness.

Where the tests could be misleading, masks – especially double masks and N95 versions – will help, which makes it crucial for people to follow the second part of the Indian guidance.