Why Covid-19 Testing Matters

The first known case of Covid-19 in the United States was confirmed on January 20, 2020. To identify the virus, scientists took a mucus sample from a man in Washington State who had come down with a cough and fever. Then they flew that sample to a government testing lab in Atlanta, Georgia, where the infection was confirmed. Since then, SARS-CoV-2 (the virus that causes Covid-19) has infected more than 32 million Americans, and more than 153 million people worldwide. In that time, testing has become a critical tool in the fight against the extremely contagious disease.

As of late April 2021, nearly 400 million Covid-19 tests had been performed in the U.S., according to the Centers for Disease Control and Prevention. “Tests quickly tell us where infections are brewing,” says Dr. Cassandra Pierre, assistant professor of medicine at Boston University and a physician at Boston Medical Center. “Then we can introduce strategies like quarantining to prevent infections, hospitalizations, and deaths.”

Read on to learn about the science behind Covid-19 testing—and the important role testing plays in controlling the disease’s spread.

At the beginning of the Covid-19 pandemic, a nasal swab test was created to detect whether a person was infected with the virus. To perform the test, a technician inserts a cotton-tipped stick that’s about 15 centimeters (6 inches) long—or about the length of a pencil—into the back of a person’s nose to collect mucus. “This is because the virus likes to gather far back in the nasal passage and at the back of the throat,” explains Pierre.

The procedure is safe and over quickly, says Dr. Jaimie Meyer, an associate professor of medicine and public health at Yale University and a doctor of infectious diseases at Yale Medicine in Connecticut. “It takes two seconds, and it feels like that tickle you have right before you sneeze.”

In addition to the nasal swab test, there’s also a newer saliva-based alternative. It requires patients to spit into a small plastic tube until enough saliva is collected for testing.

Once collected, the mucus or saliva sample gets tested in one of two ways (see Covid-19 Tests Compared, below). One test uses a technique called polymerase chain reaction, or PCR. If the SARS-CoV-2 virus is present in the sample, PCR will make millions of copies of its genetic material. That way, scientists will be able to easily detect the presence of SARS-CoV-2 in a sample, even if only a tiny amount of the virus is present.

PCR tests are nearly 100 percent accurate. But they can also be costly and time-consuming. It can take two to five days to process each sample.

The other type of test detects specific viral protein molecules, called antigens. A patient’s saliva or mucus sample is mixed with a chemical that breaks open the virus to release these proteins. This sample is placed on a test strip that contains antibodies. These proteins react with those from SARS-CoV-2. If a dark band or bright glow appear on the strip, that means the patient who provided the sample has tested positive for Covid-19.

An antigen test is faster, simpler, and less expensive than a PCR test—the process can take only 15 minutes. Unfortunately, an antigen test is also less sensitive than a PCR test. For the antigen test to work, the saliva or mucus sample must contain enough virus particles to react with the antibodies on the paper strip. If a sample doesn’t contain a high number of particles (like one taken from an infected person too soon after they were exposed to Covid-19), an antigen test may give a false negative.

The first vaccines that aim to prevent people from becoming ill with Covid-19 were authorized for emergency use in the U.S. late last year (see U.S. Authorizes First Vaccine for Covid-19). About 30 percent of adults have been fully vaccinated so far. Government scientists say that 70 to 80 percent of the population needs to be vaccinated to effectively stop the virus from spreading. Until that happens, testing for Covid-19 can help minimize transmission.

If anyone has symptoms of Covid-19, like a fever or a dry cough, public health experts are recommending that they isolate from others and get tested. This is true for vaccinated people too, since none of the authorized vaccines are 100 percent effective. Even if somebody is not experiencing symptoms, the CDC recommends that they get tested if they’ve been in close contact with someone who tested positive for Covid-19 or if they’ve been in a crowded indoor place where social distancing was difficult, like an airport or a large gathering.

If the test result is positive, it’s recommended that the person isolate for 10 days after their symptoms first appeared. If a test result comes back negative, it’s still important to be cautious, says Dr. Hilary Godwin, dean of the University of Washington’s School of Public Health. Because of the possibility of false negatives from antigen tests, Godwin recommends following up with a more sensitive PCR test to confirm a negative result. Meyer agrees: An antigen test will give the fastest results, she says, but “for the best and most accurate answer, a PCR test is the way to go.” To reduce the risk of a false negative, both scientists say the best time to get tested is three to five days after being in a high-risk situation.

“We are at an in-between place in the pandemic,” says Godwin. “Some people are [fully] vaccinated, but many are not. During this time, testing is still essential to help us identify which people should isolate and quarantine to prevent further spread of the disease.”