Just how easily does the delta variant spread?

It lasted only a few seconds: One man walked past another man in a mall in Sydney. When officials later watched CCTV footage of the encounter, they saw that this was the only interaction between the two. But it was enough for one of the men, who didn’t realize that he was infected with the delta variant of the coronavirus, to infect the other.

As COVID-19 cases from the delta variant continue to swell, it’s clear that some things are fairly risky—it isn’t an ideal time to spend hours sitting unmasked in a crowded bar, especially with unvaccinated people. But how risky is it to quickly pass someone at a grocery store or wait in line at a bank?

It makes sense that spending more time with someone poses more risk, though worst-case scenarios like the one in Sydney show that it’s technically possible for the virus to be transmitted even during a fleeting encounter. “The challenge is that, of course, the amount of an exposure that’s required to cause an infection is always going to vary,” says David Dowdy, associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health. “It’s always going to be possible for a very fleeting exposure to cause an infection. But it’s always going to be much, much more likely for a very intense exposure to cause an infection. It’s the same thing as saying, ‘Can you get the flu from passing someone on the street and they sneeze? Yes, of course you can.’ But how many people do you pass on the street, and how often do you get the flu from them, versus if your child gets the flu [and infects you]?”

The question is difficult to answer because little data exists. As of a year ago, in the U.S. nearly five of six cases of COVID-19 went undetected, according to a recent study from the National Institutes of Health that looked at blood samples. Even when people get tested and realize they’re infected, contact tracing is sporadic at best. In parts of the country last year, more than half of people with COVID-19 declined to share their contacts. When infections surge, contact tracers also often don’t have enough resources to keep up. And it gets harder to discover which person infected others if a large percentage of people in an area are sick.

In July, some people were infected at an outdoor stadium in Melbourne; they hadn’t been sitting next to the people who were infected or otherwise appeared to have close contact. An epidemiology professor in Melbourne told a local news outlet that it’s likely that this type of infection from a fleeting encounter wasn’t limited to the delta variant. “We’re now working out some of these transmissions because the contact tracing is so much better . . . but it was also probably happening last year,” he said.

Others have suggested that it’s more likely to happen with delta than with the original virus. The Centers for Disease Control and Prevention had originally estimated that “close contact” with an infected person meant spending 15 minutes together, unmasked and indoors. A recent, not-yet-peer-reviewed study suggests that the delta variant may have a viral load that is 1,000 times higher than the original virus. If that’s correct, Céline Gounder, a clinical assistant professor of medicine and infectious disease at NYU’s Grossman School of Medicine, roughly calculated that the equivalent amount of time for the delta variant would be one second of contact rather than 15 minutes. Still, 15 minutes was never a magic number—the CDC later revised it to say that a cumulative 15 minutes throughout the day would also be considered close contact, and it’s likely that some people were unlucky enough to catch the original virus much faster.

Whether someone gets sick from these exposures also depends on many variables, including the state of their own immune system, how long the infected person has been sick and their personal viral load, the ventilation in the space, and other factors. Another worst-case scenario is a transmission documented in a Korean hospital where one person got sick after going into a shared bathroom 40 minutes after an infected person had used it. But the publicly available government reports on the cases in Australia omit some important details, including whether the people were wearing masks (they likely were not, as they weren’t required to at the time) or whether the people who caught the virus were vaccinated (they likely were not, as Australia has been dealing with vaccine supply issues).

The bottom line: It’s crucial to get vaccinated. There’s still a chance you’ll get sick, but it’s less likely, and it’s much less likely that you’ll end up severely ill or dead. “I think that for people who have not yet been vaccinated, there’s no better time,” Dowdy says. “The further we go on in this pandemic, the more we learn that these vaccines are very safe. And the more everyone’s risk goes up, the greater the benefit of getting the vaccine. I think that it’s also just important for people to realize that cases are going up, it is time to be a bit more cautious, to really think about the level of exposure that we are all undertaking. If we make some small changes and reduce those personal levels of exposure, we can go a long way to making the peak of this current wave much less severe than it otherwise would be.”