Swabbing the nose with a rapid antigen test will not reliably detect the Omicron variant in the first few days of an infection, so manufacturers should seek US approval to allow users to safely collect samples from the throat as well, according to an infectious diseases expert.

The US Food and Drug Administration has expressed concerns over the safety of self throat swabbing.

People can already transmit Omicron to others when it has infected their throat and saliva but before the virus reaches their nose, so swabbing the nostrils too early in the course of infection will not pick it up, Dr. Michael Mina, formerly of the Harvard T.H. Chan School of Public Health and now chief science officer at eMed, said during a news conference on Thursday.

A study released on Wednesday on medRxiv ahead of peer review looked at 29 Omicron-infected workers in high-risk professions who had PCR and antigen tests done simultaneously on multiple days. The PCR tests of saliva detected the virus on average three days before the rapid nose-swab samples became positive.

"When people test negative by rapid antigen, they can still have very infectious viral loads and transmit to other people," said study leader Blythe Adamson of New York-based risk reduction company Infectious Economics LLC.

A positive antigen test is very reliable, Mina noted.

On social media, some experts have advised antigen-test users to swab the throat before swabbing the nose. The FDA, however, said users should follow manufacturers' instructions.

The agency said there are safety concerns regarding self-collection of throat swabs, "as they are more complicated than nasal swabs, and if used incorrectly, can cause harm to the patient."

"The CDC recommends that throat swabs be collected by a trained healthcare provider," the FDA said.

Before the FDA would approve at-home throat swabbing, manufacturers would need to conduct studies to show that consumers could safely perform the tests, and provide detailed instructions with each test kit, Mina said, adding that people go against the FDA's advice "at their own risk."

He noted that testing kits provided to consumers in the UK do include instructions for swabbing the throat. While it is important to follow the FDA's guidance, he said, "we also need to follow the science."

Research shows that COVID-19 symptoms appear earlier with Omicron than with other variants.

"When you feel symptoms, assume you're positive," but wait to use the antigen tests until a few days have passed, advised Mina, whose company develops technology to verify antigen test results.

The tests "are detecting Omicron just fine" once it has invaded the nose, he said.