Pick up a virus

This in turn appears to be linked to how likely the virus is to be infectious - tests that have to go through more cycles are less likely to reproduce when cultured in the lab.

But when you take a coronavirus test, you get a "yes" or "no" answer. There is no indication of how much virus was in the sample, or how likely it is to be an active infection. A person shedding a large amount of active virus, and a person with leftover fragments from an infection that's already been cleared, would receive the same - positive - test result.

But Prof Heneghan, the academic who spotted a quirk in how deaths were being recorded, which led Public Health England to reform its system, says evidence suggests coronavirus "infectivity appears to decline after about a week".

He added that while it would not be possible to check every test to see whether there was active virus, the likelihood of false positive results could be reduced if scientists could work out where the cut-off point should be.

This could prevent people being given a positive result based on an old infection. And Prof Heneghan said that would stop people quarantining or being contact-traced unnecessarily, and give a better understanding of the current scale of the pandemic. Public Health England agreed viral cultures were a useful way of assessing the results of coronavirus tests and said it had recently undertaken analysis along these lines. It said it was working with labs to reduce the risk of false positives, including looking at where the "cycle threshold", or cut-off point, should be set.

But it said there were many different test kits in use, with different thresholds and ways of being read, which made providing a range of cut-off points difficult.

The testing kits come as thousands of LA Unified School District students and staff are expected to head back to classes Tuesday. Once residents perform the test, they can return the completed kit to a designated location for processing, and be notified of their results within 24 to 48 hours. County officials say they are not intended to be used as a requirement to return to school, work, or participation in another activity where routine testing is required.

This could mean the virus transmits more easily, while only causing mild or asymptomatic disease. Scientists do not yet know whether omicron is more infectious than other variants, whether it causes more severe disease or whether it will overtake Delta as the most prevalent variant. It may take several weeks to get answers to these questions. Cells in the lungs and in the gastrointestinal system can harbor SARS-CoV-2 and common-cold coronaviruses simultaneously, according to earlier studies.

In the meantime, Soundararajan said, the new findings underscore the importance of people getting the currently available COVID vaccines. Keep up with the latest medical breakthroughs, healthcare trends, pharmaceutical news and more. Subscribe to our daily curated newsletter to receive the latest exclusive Reuters coverage delivered to your inbox. More from Reuters. Global Healthcare Keep up with the latest medical breakthroughs, healthcare trends, pharmaceutical news and more.