It appears the coronavirus is mutating in the U.S.

Researchers became aware of this mutation when Houston saw an influx of cases in June, where a form of COVID-19 that was found in other parts of the world and was known to be particularly contagious began to dominate the city’s confirmed cases.

A new study covered by the Washington Post analyzed the phenomena by comparing virus samples from two different COVID-19 waves from the Houston area in the spring and summer. While the city’s residents were exposed to multiple strains, almost every sample from its larger outbreak in late June was of the mutation found in cases in Europe.

Patients with this particular strain of coronavirus bore more viral particles than other people, which meant they were probably more infectious. Experts say cases in Houston spiked due to the contagious strain, where new cases surged from around 200 a day to over 2,400. The mutation doesn’t seem to make the virus more deadly or alter patients’ outcomes. Another study showed that the same strain was dominant in the U.K. during the spring.

“We’ll have to chase the virus and, as it mutates, we’ll have to tinker with our vaccine,” David Morens, a virologist and advisor at the National Institute of Allergy and Infectious Diseases, told the Post.

Scientists aren’t surprised that the virus has evolved since it has had ample opportunity to mutate. And while most mutations have no effects, they can alter the virus’ behavior or makeup—and if that helps the virus’ transmission, it can allow the mutated version to dominate. If the virus develops over time, then that could impact the efficacy of a vaccine, which might need to change as the virus evolves.