Some 4.8 million doses of flu vaccine are now being prepared for use in response to the growing U.S. outbreak of highly pathogenic avian influenza, or H5N1, officials say. 

The order for the doses to be filled into vials comes as the Centers for Disease Control and Prevention is investigating a growing number of human cases linked to the unprecedented outbreak of the virus in dairy cattle this year. 

The supplies will be pulled from a "pre-pandemic" stockpile funded by the federal Administration for Strategic Preparedness and Response. Vaccinemaker CSL Seqirus has been tasked with manufacturing the doses at its North Carolina plant.

"It utilizes a highly scalable method of production and is currently positioned to deliver up to 150 million influenza vaccine doses to support an influenza pandemic response within six months of a pandemic declaration," CSL Seqirus said in a release.

The doses combine bulk stockpiles of two key ingredients: an "antigen" targeted at the H5 portion of the H5N1 virus with an "adjuvant" designed to boost the immune response triggered by the vaccine.

While manufacturing of these new bird flu doses is set to be completed by "later this summer," the vaccines still may not be immediately ready for use.

"I do want to decouple availability and manufacturing completion, because there is of course that regulatory component that would be needed to either approve or authorize actual use of the doses," ASPR's David Boucher told reporters Thursday.

The FDA has previously approved other vaccines to prepare for potential H5N1 pandemics, including a vaccine by Seqirus. It is unclear how soon the FDA could grant authorization to clear use of the new shots. 

"While CDC's current risk assessment to the general public is low, if it is determined that the U.S. population needs to be vaccinated to prevent H5N1 influenza, then the FDA will use its regulatory pathways to take the appropriate steps to ensure vaccines are available in the timeliest manner possible," an FDA spokesperson said in a statement.

Boucher also said that authorities have so far not decided whether to recommend use of the vaccines. It is unclear who would be prioritized for the first shots if they are eventually deemed to be needed and rolled out for the public.

"If that changes in the future, we would be looking at the population for whom vaccination would be recommended and then making any decisions on prioritization at that time," said Boucher.

A panel of the CDC's outside vaccine advisers, which has discussed previous pandemic vaccines, is scheduled to meet in June to discuss H5N1 alongside its routine votes on recommendations for seasonal flu vaccines. 

While the CDC says the risk to the general public from H5N1 remains low for now, the agency has warned that workers in places like dairy farms and production facilities are now at higher risk of infection and it has urged the industry to take precautions.

At least three workers at U.S. dairy farms have tested positive this year after working directly with infected cows. 

The latest case in Michigan developed respiratory symptoms, officials announced Thursday, which could make it easier for the virus to spread from person to person. 

"Simply put, someone who's coughing may be more likely to transmit the virus than someone who has an eye infection like conjunctivitis," the CDC's Nirav Shah said Thursday.

However, Shah said there is no evidence that person-to-person spread has happened so far. Contacts of the patient have so far not tested positive. Previous H5N1 spillovers around the world from different strains have also fizzled out after limited spread between people.

The CDC is now looking to see if the strain that infected the latest Michigan patient might have developed key mutations that could raise its risk of spread.

"The mere fact that this individual displayed some respiratory symptoms, again we should be alert, but in and of itself is not a cause to change course or suggest that we're at an inflection point," Shah said.

In:
• Bird Flu
• Avian Influenza

Alexander Tin

Alexander Tin is a digital reporter for CBS News based in the Washington, D.C. bureau. He covers the Biden administration's public health agencies, including the federal response to infectious disease outbreaks like COVID-19.

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