Before Christmas, health experts suggested getting a rapid coronavirus test just before any rally.

But some health experts are now warning that you can test negative even if you are infected and contagious while still being visibly healthy.

In general, the tests are able to reveal an infection with Omicron, but enough viruses must have reproduced and appear at levels high enough in the nose or saliva to be detectable, according to Dr. Michael Mina, epidemiologist and former professor. at Harvard TH Chan School of Public Health, whose interview on the “In the Bubble” podcast, hosted by former White House COVID-19 adviser Andy Slavitt, was published this week.

“Omicron appears to be more infectious, so it could take off and spread on the first day or two before there is enough virus in your nose to transform the [rapid] antigen test positive – or PCR test positive, for that matter, “Mina said on the podcast. “You might already be infectious, and that’s potentially because the virus is now so capable of aerosolizing and getting out of people in lower amounts.”

The United States Food and Drug Administration has long warned that rapid antigenic tests “may fail to detect early coronavirus infection.” But those warnings hadn’t received much attention in the months leading up to Omicron’s push.

People can be contagious whether or not they have symptoms, and many people infected with the coronavirus will not have any symptoms.

People infected with the coronavirus can still test negative even after starting to show signs of symptoms of COVID-19, with subsequent tests over the next two days eventually turning positive.

This happened recently with the 28-year-old son of Dr. Robert Wachter, chairman of the medical department at UC San Francisco.

He wrote recently that her son, who had received three injections of the Moderna vaccine, became symptomatic two days after watching a movie at home with a friend. The vaccinated friend subsequently tested positive, three days after the movie night.

The son’s first rapid test, taken two days after the movie night, was negative, although he had symptoms such as a sore throat, dry cough, muscle aches and chills, but no loss of taste or of smell.

After finding out that his friend had tested positive, the son had another rapid test, which came back positive. For this test, a swab was used in his throat before it was placed in his nose.

It’s “a little disgusting,” Wachter said, but it was done amid the first data suggesting that Omicron starts showing up at higher levels in the throat before starting to show up at detectable levels in the nose. . (British health authorities have produce a video on how to do it.)

Wachter’s son is feeling better, with a mild sore throat and no fever five days after the onset of symptoms, Wachter noted Monday.

For people who show signs of illness, symptoms seem to start much sooner after exposure than with earlier variants, Mina wrote.

“This means that there is a chance that the virus will not yet develop in the nose on the first test,” he said.

Some people have expressed concern that rapid nasal swab tests may not detect coronavirus as early as rapid saliva swab tests. Mina said nasal swabs still catch the virus on most days a person is contagious and are still useful in stopping transmission.

“Just like a seat belt, these are all risk mitigation strategies – they are not risk erasure strategies. And so, just like a seat belt and an airbag, neither of them are perfect, ”Mina said on the podcast.

Among people who contract the Omicron variant and eventually show symptoms, signs of illness may appear just one to two days after exposure, with contagiousness starting about two days after exposure, Mina said.

And symptoms of a person who has been vaccinated or who has been previously exposed to the virus may appear earlier than a person who has not been vaccinated or who has not been infected. It’s because their immune system has learned to recognize the virus, Mina said.

The first symptoms can be congestion, runny nose, and fever, which are consistent with the immune system quickly recognizing the virus and attacking it.

In contrast, the immune system of a person who has not been vaccinated or who has not been previously exposed to the coronavirus would not be able to recognize it. In the pre-Omicron phase of the pandemic, it typically took five to seven days for symptoms to appear.

The earlier onset of symptoms after exposure to the coronavirus is a good development, Mina said. Before there was so much exposure to the coronavirus, many infected people who eventually developed symptoms were contagious for two to three days before starting to feel sick.

There is a potential blind spot of a day in which PCR and antigen tests are unable to detect the contagious status of a person positive for the coronavirus.

“Probably the first day that you are contagious with Omicron, you are probably going to have a negative on [a rapid] antigen test, ”Mina said. When it comes to a PCR test, there’s a 50% chance that the test result will be positive on the first day you’re infectious, Mina said, but even then the results wouldn’t be available for a few days at a time. cause of laboratory treatment. time.

“On average, everyone should assume that there could be up to a day before a test tells you you’re positive,” Mina said.

The first day you are contagious, you probably won’t be “super contagious”. On the second day of contagiousness, however, “you might actually become quite contagious,” Mina said. A rapid test will likely always be negative; a PCR test may be positive, but this result may not be available for a few days.

It could take up to the third day of contagiousness before a rapid test is positive, Mina said.

Rapid tests are always useful to determine if a person is contagious on most of the days that they are able to transmit the virus. On the third or fourth day of infectivity, when the level of virus produced by the body is astronomical, rapid tests can detect high levels of the coronavirus, Mina said.

For tests that show you are no longer contagious, a PCR test isn’t helpful, experts say. They can show positive test results for weeks after an infection, long after a person has stopped being contagious.

A rapid test begins to show negative results by the time the contagiousness ends, Mina said. ” If you do not have [contagious levels of] virus, the antigen test tends to turn negative, and if you do, the antigen test tends to stay positive, ”Mina said.

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