The explosion of omicron cases in the U.S. smack in the middle of the holiday season is forcing Americans to rely more heavily on rapid tests prior to congregating with friends and family. In some select cases for people who are vaccinated, however, test results might not be as straightforward as they seem.
There have been anecdotes of breakthrough cases in which people experience symptoms before they get a positive result on a rapid test. In other words, you could develop a dry cough, muscle pain, and nasal congestion but still initially test negative. If you were to test days, or even hours, later, that result could eventually turn positive. A study by former Harvard epidemiology professor Michael Mina, who also posted a Twitter thread on the matter, explains how this could happen:
Basically, the vaccines enable the immune system to detect the virus more quickly. Symptoms may therefore appear earlier, when levels of the virus are still too low to be detected by a rapid test. The discordance between symptoms and test results may seem scary, but it’s a promising sign that the vaccine has effectively fortified the immune response. “In some vaccinated individuals, the virus may elicit an immune response that prompts the onset of symptoms before enough virus is present to be detected by an antigen test,” said Amesh Adalja, a professor at the Johns Hopkins Bloomberg School of Public Health. “So, the symptoms may be present but not enough viral protein to detect. At that point, however, the person is not yet contagious and possibly could be detected by PCR test.” In fact, the U.K. is currently advising that people with symptoms use PCR tests rather than rapid tests.
This lag between symptoms and positive results is partly due to the relative imprecision of rapid tests, also known as lateral flow tests. Rapid tests are designed to detect the viral proteins that appear when you’re infectious, and you typically need higher levels of the virus present in the nostrils to get a positive result. “You need quite a lot of virus. The fact is that lateral flow tests are only as good as detecting virus when it’s present on something like a swab,” said Al Edwards, a biomedical technology professor at the University of Reading. “If you swab yourself, if there’s plenty of virus, the lateral flow test will be positive.” PCR tests, on the other hand, detect the presence of genetic material from the virus. They’re considerably more accurate because the processing of the test involves a procedure in which the genetic material is multiplied, which means that it can detect even a small amount of the virus. Yet, because most PCR tests have to go to a lab for processing, it usually takes three to five days to get a result, so it’s fairly impractical to use them for regular screenings. You can take a rapid test at home and get a result within 10 to 15 minutes, so they’re more useful for asymptomatic people who are trying to incorporate testing into their day-to-day routines. Rapid tests are also generally reliable when it comes to figuring out if you’re contagious, so they can be helpful right before you meet up with other people indoors.
Ultimately, if a rapid test returns a positive result, you should take that seriously. But if it’s negative, you might take that with a grain of salt, particularly if you’re showing symptoms. Definitively determining that you don’t have the virus is a difficult task. “At the moment, there’s no test that exists that can guarantee that you’re not infected,” said Edwards. If you have symptoms and get a negative result, though, it may be worth trying a different brand of rapid test or a PCR. (The FDA has found that certain tests from the diagnostics companies Meridian Bioscience, Tide Laboratories, and Applied DNA Sciences may be unable to detect the omicron variant.) Adalja advised, “If it’s negative, you still need to figure out what you’re sick with. It may be COVID and might need a different type of test, or it could be some other infection.”