Amid a US outbreak of avian influenza in poultry flocks and dairy cattle, the federal government on Friday announced $72 million in funding to three vaccine manufacturers to expand the production of bird flu vaccines for humans, in the event that they are needed.

The H5N1 virus has affected millions of wild and commercial birds nationwide, and in March it made the jump to dairy cows for the first time. As the number of affected animals grows, so does the concern for spread to people. In the past, H5N1 has had a high mortality rate in humans, and scientists are monitoring the virus closely to determine whether it poses a pandemic risk. The US government has a stockpile of approved H5N1 vaccines, but today’s awards, which will go to CSL Seqirus, GSK, and Sanofi, will double that number.

“We do expect that we will have a total of just over 10 million doses filled and finished by the end of the first quarter of calendar year 2025,” said David Boucher, director of infectious disease preparedness and response at the Administration for Strategic Preparedness and Response, in a press briefing on Friday.

Moderna, Pfizer, and GSK are also working on mRNA vaccines for bird flu, but those need to go through human testing and be approved by the US Food and Drug Administration before they could be used.

This year, 16 people in the US have been infected with bird flu. The US Centers for Disease Control and Prevention confirmed the latest two cases, both in California, Thursday evening. The individuals are farm workers who had contact with infected dairy cows at two facilities in the Central Valley, the epicenter of the state’s cattle outbreak. Both had mild symptoms, including eye redness, and are being treated with antiviral medication. Neither reported respiratory symptoms or were hospitalized.

Nirav Shah, the CDC’s principal deputy director, said the new cases do not change the agency’s risk assessment for the general public, which continues to be low. “Finding these two cases was not unexpected,” he said during Friday’s briefing. “As there are more herds that test positive, there are more workers who are exposed, and where there are more workers who are exposed, the chances of human infection increase.”

Both California cases are thought to be instances of animal-to-human spread, with no known link or contact between the two. California health officials are following up with close contacts of the workers, including household members that are also dairy workers.

In the coming days, the CDC says it will do additional testing on the virus samples, including genetic sequencing to monitor for any changes in the virus. Scientists are particularly interested in whether the virus is developing new mutations that could make human-to-human transmission more likely. The agency says there is no evidence that the virus can spread from person to person at this time.

Of the 16 confirmed human cases, six have been linked to exposure to sick or infected dairy cows, while nine had exposure to infected poultry. The source of infection for a case in Missouri has not been determined.

The Missouri case, confirmed in September, was detected through routine flu surveillance and is the first known case of human bird flu in the country with no known exposure to a sick or infected animal. In late August, the person was hospitalized in Missouri due to underlying medical conditions. The patient did not have respiratory symptoms and was not severely ill but tested positive for flu as part of routing screening.

The Missouri State Public Health Laboratory determined that the patient’s virus wasn’t a seasonal flu virus and forwarded the sample to the CDC, which confirmed it was a type of bird flu, or H5.

Out of an abundance of caution, health officials carried out a retrospective investigation that identified 112 health care workers who had contact with the patient. Of those, six people reported respiratory symptoms around the time the patient was hospitalized. Those symptoms could be due to other viruses, such as seasonal flu or Covid-19. Only one of the health care workers had active symptoms at the time of the investigation and tested negative for flu on a PCR test. The CDC collected blood samples from the health care workers to look for the presence of bird flu antibodies, which would indicate a prior infection.

Demetre Daskalakis, director of the CDC’s National Center for Immunization and Respiratory Diseases, says the H5 antibody test is complex to perform and that results may not be available for another two weeks.

The CDC is also trying to reconstruct the virus’s genetic sequence from the hospitalized patient’s sample, which could reveal clues about its source. Analyzing the virus has proven difficult, because only a tiny amount of viral genetic material could be isolated from the patient sample.

In the absence of vaccines specific to bird flu, health officials are encouraging poultry and dairy workers and other high-risk people to get a seasonal flu vaccine to help protect themselves.