Traveling this pandemic? A new study showed that testing for COVID right before leaving is the best thing to do.

For the study published in the International Journal of Public Health, the team of epidemiologists from Yale School of Public Health were the first to use statistical analysis to figure out how effective test-timing strategies are at limiting COVID-19 spread. The team found that testing on the day of an event/outing can nearly halve the risk of transmitting the virus.

The team also stated that policies requiring people to have a negative COVID test within 72 hours or more on arrival at a location – something most countries require for travelers – are hardly helpful.

According to senior author Jeffrey Townsend, the Elihu Professor of Biostatistics at the Yale School of Public Health, this is because the virus grows exponentially in the human body. Taken early enough, a rapid-antigen (RA) test can fail to detect faint traces of the virus before it builds up enough to result in a positive test. This means in just a matter of hours, someone unknowingly infected but tested negatively could spread the virus to other people.

“Typically, the disease has a very short period where it is really highly transmissible. 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,” said Townsend.

To understand how transmission changes with testing at different times, Townsend and his colleagues applied mathematical modeling. Per the study, the probability that a person with COVID would transmit the virus to one or more individuals after the start of an event is roughly 40% without any testing. However, as testing is performed closer and closer to arrival time, that risk drops more and more.

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

As part of their analysis, the team analyzed more than a dozen different RA home tests and found that they are more effective than RT-PCR tests since they provide quick results.