Faced with the rapid spread of avian flu through California dairies, health officials are now doing weekly testing of every dairy farm in the state — expanding initial efforts amid new evidence that some infections are going undetected and there may be unknown paths of transmission.

But farmworkers are not being vaccinated, unlike workers at poultry and fur farms in Finland. Why not?

On Friday, health experts offered updates about what we’re learning about the virus.

Q: The federal government has vaccines that protect against H5N1, the virus that causes avian influenza, in its Strategic National Stockpile. Why aren’t we using them?

A: The virus isn’t spreading between people, said Erica Pan, California’s state epidemiologist. California’s cases have been mild. And antiviral oral medications are effective against the virus. “We’re remaining proactive, if things change,” she said.

Q: Do this year’s flu vaccines help protect us?

A: There’s not enough information to know if our existing flu vaccines will protect us, said microbiologist Dr. Bobbi Pritt of the College of American Pathologists. It’s unlikely; this year’s vaccine is designed to fend off the two circulating subtypes of influenza A and the one circulating subtype of influenza B — not avian flu.

Scientists are designing a vaccine for cattle, which will help reduce exposure risk, said Dr. Ben Bradley, also with the College of American Pathologists. But it will be impossible to vaccinate wild birds.

Q: California has 36 confirmed human bird flu cases. But could some people be asymptomatic, so they’re missed?

A: So far, the state is focusing only on cases of people with known illness. Wastewater surveillance is a way to monitor the virus — but it mostly catches virus that is shed by flying birds. Also, there is increasing evidence that wastewater holds fragments of dead virus from milk that we pour down our drains.

Some people may be asymptomatic or only mildly ill, so they don’t bother going to a doctor, Bradley said. And it’s tough to detect live virus in people. That’s because nose and throat swabs, used to find COVID, don’t always catch it. Most of the California cases have been found by doing an eye swab.

“California has a very robust public health testing program. Not all states have an equally robust program,” said Dr. Donald Karcher, president of the College of American Pathologists. “So it’s very likely we’re missing cases in other parts of the country.”

Q: Why does the virus cause severe respiratory illness in some people — and mild illness, like conjunctivitis, in others?

A: There are two strains, Pritt said. The D1.1 genotype, which is seen in birds, caused very severe disease in an older person in Louisiana, as well as a teenager in Canada. The B3.13 genotype, seen in cows, caused mild disease in dairy workers.

“At this point, the B 3.13 strain doesn’t seem to be associated with severe disease, but we’ll have to keep an eye on that,” Pritt said.

Illness may be influenced by the route of infection, Bradley added. Dairy workers’ eyes may have gotten a splash of infected milk in their eyes.

Q: Dairies are frustrated because they’re taking protective steps, but still get infections. What’s going on?

A: “We don’t know. That’s what the research is targeting,” said state veterinarian Dr. Annette Jones. “It just seems like something else is causing the spread as well.” Perhaps asymptomatic but infected new cattle are brought onto farms. Or maybe someone neglected to clean their footwear before work.

Cows can be asymptomatic for a number of weeks, she added, so may inadvertently transmit disease.

Q: Is avian flu affecting the state’s milk supply?

A: Even if a dairy has sick cows and is quarantined, cattle don’t typically die from the disease. Unlike birds, they recover, Jones said. Once the virus is no longer detected, the dairy can resume selling milk.

Q: Are eggs safe?

A: Because chickens show signs of disease so quickly, and die, officials can ensure that eggs from infected flocks don’t enter the market, Jones said. Additionally, sick hens don’t tend to transmit the virus to their eggs.

Q: Why is the illness so different in cattle and birds?

A: In birds, the D1.1 strain is well studied, and known to be highly contagious. And it is seasonal, with poultry infected by migrating waterfowl. This fall, 51 commercial farms and nine backyard flocks in 13 counties have been affected. It is also very severe. Flocks must be killed to ease their suffering and reduce the risk of spread. “It’s basically a death sentence for a poultry flock,” said Jones.

The virus just jumped into cows this spring, so there’s a lot we do not know about how it behaves. So far, it isn’t seasonal. This B 3.13 strain doesn’t sicken cows as severely as it does poultry. Only 1% to 2% of cows die. So the state’s response is different, focusing on containment, not euthanization. So far, 679 of 984 dairies have been quarantined. Of these, 66 are once again virus-free.

For the first time, officials found the cow strain in poultry flocks. They don’t know why. Perhaps rodents are tracking in from one farm to another.

Q: When should we start getting worried about an outbreak among humans?

A: Most worrisome are the two cases in the U.S. with no clear link to cows or birds — one in Michigan, one in Oakland, Bradley said. “If we would be seeing more of those cases,” he said, “that’s something that would lead me to be concerned.”

There are also concerns because studies of blood show antibodies to prior exposure, suggesting that there are asymptomatic cases and the real number of infections is higher, he said.

Another bad sign would be a jump in flu cases in the summer, outside the traditional flu season, indicating a spread.

“If we’re seeing more human adaptation, that’s going to raise red flags,” Bradley said. “It says: ‘This is something we need to be more aggressive in testing for.'”

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