CDC: Human Risk from H5N1 Bird Flu is Low | ID Week 2024

The public health threat from H5N1 — the avian influenza A virus that is currently infecting dairy cows, poultry and other animals in the United States — is low. But we should continue to expect sporadic transmission from animals to people, especially among farm workers, Timothy M. Uyeki, M.D., MPh., MPP, chief medical officer, Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention (CDC), said in a presentation at ID Week 2024, which is being held this week in Los Angeles.

Timothy M. Uyeki, M.D.

“There’s no evidence of viral changes associated with increased transmissibility to people,” he said. “We need to especially protect workers from occupational exposures, and we need to control the spread in farmed animals. We can’t control the spread in wild birds. We can try to control poultry outbreaks, but we need to do a better job at controlling the spread in farm animals.”

Since April 2024, there has been 20 cases of H5N1 infection in people in the United States; 21 in total since 2022, according to data released recently by the CDC. Ten of these cases were from exposure to infected poultry and nine were from exposure to infected dairy cows. Among animals, the USDA is reporting that 299 dairy herds in 14 U.S. states have confirmed cases of H5N1 bird flu virus infections in dairy cows.

Human infection with H5N1 can cause mild-to-severe disease, with conjunctivitis being a common early symptom among dairy and poultry farm workers in the United States. The incubation period is three days, and the virus can result in fever, cough, headache and gastrointestinal symptoms. It can progress to pneumonia and respiratory failure. Other complications include acute kidney failure, cardiac failure, sepsis and shock.

The good news about the virus circulating in 2024 is that it doesn’t appear to be airborne and appears to respond to Tamiflu (oseltamivir), a commonly used treatment for people with influenza A and B. However, there are no clinical trials for therapies specifically to treat H5N1 and there is limited observational data, Uyeki said.

“There is a gap in knowledge,” he said. “It’s hard to make recommendations [for treatment]. We have in vitro data, and we have some in vivo data in mice, but we don’t have data on the clinical management in those with severe disease. That is why we are recommending infection prevention control measures.”

He did say high-dose corticosteroids are not recommended because they are associated with prolonged viral shedding and might increase the risk for ventilator-associated pneumonia and death.

Uyeki also cautioned that should the virus change, and especially if it begins to infect pigs, the risk to people could be higher. “This could change very suddenly, and so we really need vigilance and enhanced virologic and disease surveillance, but not just for H5N1 but also other novel influenza A viruses. And we need to focus on swine, because if H5N1 viruses get established in swine, that could be a big game changer. The risk of an influenza pandemic with increased transmissibility to people would go up.”

He said that pigs are “mixing vessels” that have both human-adapted and avian-adapted receptor types in their respiratory tract. This provides a host for seasonal and avian influenza A viruses to go through a reassortment into a novel virus that is more of a threat to people.

This is what happened with the H1N1 virus epidemic in 2009. The CDC estimated that from April 12, 2009 to April 10, 2010, there were 60.8 million cases in the United States, with 274,304 hospitalizations and 12,469 deaths. Worldwide, between 151,700 and 575,400 people died from H1N1 during the first year the virus circulated. About 80% of those deaths were in people younger than 65 years of age.