Omicron could make you infectious more quickly than Delta — take a rapid test no earlier than a few hours before heading to a party, experts say

• Experts have advised people to take rapid tests for COVID-19 hours before a party, not the day before.
• A person with the new Omicron variant becomes infectious quicker than with Delta, early data suggest.

Partygoers should take a rapid test to check for COVID-19 closer to the start of an event, experts have said in response to the spread of the Omicron variant.

People who catch the new Omicron variant become infectious much quicker than those with Delta, early data from the UK indicates. Testing too early could allow for cases to be missed, experts have cautioned.

The Omicron variant, which has 32 mutations in the part of the virus that infects human cells, is spreading fast in South Africa and the UK — possibly because its mutations help it partially escape the immune response from previous infections and existing vaccines. We still don't know if Omicron will be more deadly than Delta – itself a mutated virus – which remains the most common variant worldwide, according to the World Health Organization.

Michael Mina, former assistant professor in immunology and infectious diseases at Harvard School of Public Health, said on Twitter Sunday that testing one or two days before an event "doesn't work".

"It doesn't matter the type of test, the most important thing is to test just before the event of participation," he said. You can go from "undetectable" to a "very high" number of virus particles, called a viral load, in 24 hours, he said.

Tim Spector, professor in genetic epidemiology at King's College London, said that the "transmission time" for Omicron was very short. People should take a rapid test "a few hours before leaving the house," he told the Guardian Sunday, adding that people should wear a "high quality mask" if using crowded public transport to travel to an event.

Spector, who runs the Zoe COVID-19 symptom tracker app with 4.5 million users worldwide, said on Twitter Thursday that self-reported data from the app suggested the infection time was around 48 hours. Ideally rapid tests should be taken less than 12 hours before an event, he said.

"Avoid large gatherings and split your groups up into smaller ones where you can control the environment and get everyone to test that day," Spector said, adding that people should also check no-one in attendance has had cold-like symptoms in the past three to four days, per the Guardian.

Spector said in a Tweet on December 5 that a Zoe app user attended a 60th birthday in the UK where all guests were vaccinated, some had received boosters, and all tested negative on rapid tests 24 hours before. Of the 18 in attendance, 16 reportedly caught Omicron that caused mild symptoms after the event, Spector said.

The Centers for Disease Control and Prevention recommends that people can use a rapid test regardless of vaccination status or symptoms. "A negative self-test result means that the test did not detect the virus and you may not have an infection, but it does not rule out infection. Repeating the test within a few days, with at least 24 hours between tests, will increase the confidence that you are not infected," it says.