The Centers for Disease Control and Prevention is investigating whether the U.S. stockpile of bird flu vaccines is still well matched to the virus after discovering recent mutations.

The CDC said Friday it identified genetic changes present in the virus, known commonly as H5N1, after sequencing samples from three people with the infection in California. The nation’s top dairy state has identified six human cases this month, bringing the total number of U.S. infections this year to 20. Almost all have been workers in contact with sick animals.

The viruses, which appear closely related to those currently spreading in dairy cows, contained mutations that could have an impact on the nation’s stockpile of bird flu vaccines. The CDC said it is examining whether its existing store of candidate vaccines are still well-matched to these viruses.

The U.S. has been ramping up its supply of bird flu vaccines as a nationwide outbreak among poultry and dairy cows spreads. About 300 dairy herds across 14 states have been impacted.

Though no human-to-human transmission has been identified, the government has built a repository of 4.8 million vaccine doses in case the risk the virus poses increases. It aims to have 10 million within the first quarter of 2025. It’s also supporting Moderna Inc. in its effort to develop an mRNA bird flu vaccine.

The CDC’s latest sequencing effort found no changes that increased the infectivity or transmissibility among humans. The agency believes that the immediate risk to the general public remains low, while people exposed to infected animals are at higher risk. Still, it’s increasing surveillance.

On Friday, the CDC said it was working with pharmacy networks eTrueNorth and Walgreens on a pilot program to offer influenza tests to sick patients in California and another unnamed state. If the patient tests positive, the program would determine whether additional bird flu-specific testing was needed.

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