Heading to the airport or a party? Here’s the best time to test for COVID

NEW HAVEN, Conn. — The closer to the occasion the better when it comes to COVID testing, Yale epidemiologists report. New research finds people should test for the virus on the day of an event to lower the risk of new infections as much as possible. More specifically, testing guests right at the door could cut the risk of infection at an event or party nearly in half.

This project is the first ever to use statistical analysis to measure how time of testing influences the spread of COVID-19.

Many countries currently require a COVID-19 test within 72 hours of arrival, but study authors say this timeframe does little to help curb the spread of the coronavirus. Why? SARS-CoV-2 grows exponentially in the human body upon infection. That means a rapid-antigen (RA) test, taken too early, can fail to detect faint traces of the coronavirus before it grows enough to trigger a positive test result.

In many cases, people who tested negative just a few hours prior test positive later on. Unfortunately, in these cases, these people were likely contagious the entire time.

“Typically, the disease has a very short period where it is really highly transmissible,” says senior study author Jeffrey Townsend, the Elihu Professor of Biostatistics at the Yale School of Public Health, in a university release. “Go back just a little bit of time and there’s often hardly any virus in you, compared to just a little later, when your viral load could be surging.”

The team at Yale made use of mathematical modeling to better understand how testing time intervals influence transmission rates. For example, in the absence of any testing at all, the chances of just one person with COVID-19 transmitting the virus to one or more individuals after the start of an event is about 40 percent. However, that risk level continues to decline more and more if they test closer to their arrival time.

More specifically, the study found a COVID-19 test 72 hours before an event results in a four-percent lower number of transmissions. Meanwhile, testing right before a small event can stop all transmissions entirely. At larger gatherings, tests at the door will still typically reduce the amount of COVID infections by more than 40 percent.

“We’re not the first people to say that you should test closer to an event,” Prof. Townsend adds. “But this study really nails it down: it matters an enormous amount—and here’s the curve that shows it.”

Study authors mention that if testing right before an event is impossible, tests administered 12 hours before can still help quite a bit. According to their models, COVID testing 12 hours prior to an in-person gathering can reduce the probability of COVID transmission by over 25 percent. If these tests take place 24 hours in advance, the risk drops by less than 20 percent.

Additionally, researchers analyzed over a dozen different rapid-antigen (RA) home tests. This was done to determine if test type influences transmission risk. Interestingly, they found that RA tests are actually more effective than “gold standard” RT-PCR tests in certain scenarios (such as testing right before a party) due to how quickly they produce results.

The study is published in the International Journal of Public Health.

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