Mapping an Omicron Infection – The New York Times

In less than two months, the Omicron variant of the coronavirus has spread all over the world and caused a impressive number of new infections.

Omicron now accounts for more than 99.5% of new infections in the United States, according to estimates from the Centers for Disease Control and Prevention. The nation reported up to 800,000 new cases per day in mid-January, more than three times more than at any previous point in the pandemic.

Scientists have worked overtime to study Omicron. Many questions remain unanswered, but here’s what they’ve learned so far.

Infection and incubation

Omicron moves quick. It spreads rapidly through populations and infections develop rapidly in individuals.

The time between when someone is first exposed to the virus and when they develop symptoms is called the incubation period.

Research suggests that the original version of the coronavirus and early variants had an incubation period of about five days, on average. The Delta variant appears to move faster, with an average incubation period of around four days. Omicron is even faster, with an incubation period of around three days, according to a recent CDC study.

Viral load

The amount of virus that builds up in a person’s body is known as the viral load. In general, people are thought to be more contagious when their viral load is high.

In a recent study of the Alpha and Delta variants, the researchers found that people tended to reach their peak viral load about three days after infection and clear the virus about six days after that, on average.

Whether Omicron follows the same pattern remains to be seen. In a preliminary study, the researchers found that Omicron infections were about a day shorter than Delta infections and resulted in slightly lower peak viral loads on average. But the difference could be due to higher rates of pre-existing immunity – as a result of vaccination or previous infection – among people infected with Omicron. Another research team found that among vaccinated people with infectious breakthroughs, Omicron and Delta produced similar levels of infectious virus.

Other data suggests that Omicron may not act like previous variants. Animal and laboratory studies indicate that it maybe not as good to infect the lungs as Delta, but can replicate faster in the upper respiratory tract.

The variant may also have other unique characteristics. A small study found that the antibodies produced after infection with Omicron seem protect against Delta, but Delta infections offer little protection against Omicron. If the finding holds, it means Delta could soon struggle to find hospitable hosts – and Omicron is likely to replace Delta rather than co-exist with it.

Gravity

Omicron appears to cause less severe disease than Delta. In a recent study, researchers found that people with Omicron infections were less likely to be hospitalized, end up in intensive care, or require mechanical ventilation than those with Delta infections.

One possible explanation is that Omicron is less likely to damage the lungs than previous variants. A variant that proliferates primarily in the upper respiratory tract may cause less severe disease in most people. An indication of reduced severity is that unvaccinated people seem less likely to be hospitalized with Omicron than with Delta.

But Omicron’s apparent mildness may also stem from the fact that it infects many more vaccinated people than Delta. Omicron is adept at evading antibodies produced after vaccination, leading to more breakthrough infections, but vaccinated people are still protected against the most serious disease. Booster shots of mRNA vaccines are 90% effective against hospitalization with Omicron, according to the CDC

Still, doctors have warned that although the variant may be milder on average, some patients, especially those who are unvaccinated or have weakened immune systems, may become seriously ill from Omicron infections. . And it’s too early to know if breakthrough cases of Omicron could lead long covid.

Test

Because Omicron replicates so quickly and the incubation period is so short, there is a narrower window to catch infections before people start transmitting the virus.

Earlier in the pandemic, people were advised to use a rapid test five to seven days after potential exposure to the virus. Given Omicron’s shorter incubation period, many experts now recommend doing a rapid test two to four days after potential exposure. (They also recommend getting at least two rapid tests, about a day apart, to increase the chance of picking up an infection.)

People testing to reduce the risk of transmitting the virus to others, such as at an upcoming gathering, should test as close to the event as possible, experts said.

There is still a debate about whether rapid antigen tests may be less sensitive to Omicron than other variants. PCR tests are more sensitive than rapid tests, meaning they are likely to detect the virus earlier in the course of infection, but take longer to return results.

New isolation rules

The CDC recently relaxed its isolation guidelines for people infected with the virus. Previously, the agency recommended people who tested positive for the virus stay in isolation for 10 days.

the new guidelines say infected people can leave isolation after five days if they are asymptomatic or if their symptoms resolve and they do not have a fever. People must wear tight-fitting masks for an additional five days when around other people.

The agency said these changes were motivated by data suggesting that transmission of the virus most likely occurs a day or two before the onset of symptoms and two or three days after.

But scientists have noted that some people can be contagious for longer than that, and some criticized the agency not to recommend that people receive a negative rapid test result before ending their periods of isolation.

The agency later updated its guidelines to note that people who wanted to get tested should take a rapid antigen test ‘towards the end’ of the five-day isolation period, but did not formally recommend to do.