Madeeha Hassan, Contributing Illustrator

While avian flu is currently infecting US poultry at record high levels, experts at Yale maintain that the threat to humans is still low — although public health officials should remain vigilant with surveillance and monitoring of global transmission. 

The spread of avian flu has been making headlines this year for two main reasons: first, the sheer number of bird infections detected in the U.S. has led to the surge of egg prices, and second, avian flu outbreaks have been recently detected in mammals, sparking concern of an eventual spillover into human populations. However, according to Saad Omer, director of the Yale Institute for Global Health and a professor of epidemiology at the Yale School of Public Health, while public health surveillance is important in the midst of this outbreak, the concerns to the general public should be low. 

“At this point, individuals don’t necessarily have to do anything, and not necessarily worry how they can modify their day to day actions,” Omer said. “Governments, on the other hand, and public health agencies, the more we act early, the less chance that people will have to worry.”

Influenza is a common viral infection that can affect different species, such as horses, dogs, birds, swine and people. The avian flu, or H5N1, is a strain of Type A influenza that circulates in poultry but is capable of infecting mammals as well.

According to Omer, influenza pandemics have always been a concern for public health experts because influenza viruses tend to pick up mutations relatively quickly. In addition, Sten Vermund, the Anna M.R. Lauder professor of public health at the School of Public Health, said that another reason for specific concerns around influenza is its ability to infect multiple species. 

“Simply put, the jump from birds to mammals increases the risk of humans becoming infected through a process known as ‘reassortment,’” Vermund said. “This happens when two influenza viruses, one human-adapted strain and one animal strain from birds or pigs most often, infect the same host at the same time and swap genetic information.”

Omer explained that even a new virus of modest severity among humans has the potential to cause a pandemic of considerable mortality and morbidity if there is a lack of immunity in the population. 

Thus, while human death from avian flu infection “is rare” according to Vermund, infections should still be closely monitored by public health officials.

According to the Center for Disease Control, or CDC, there have been over 58 million poultry infections and 6 thousand wild bird infections detected in the US as of February 2023. 

“What’s happening now is … almost a pandemic [in birds],” said Albert Ko, a professor of public health and epidemiology at YSPH. “It’s happening throughout many parts of the world. [It’s] a  virus of an influenza strain, which is highly pathogenic to birds, and very much highly pathogenic to poultry.”

In addition to birds, there has also been documented infection in some mammal types, including foxes, seals, stray dogs and cats, coyotes, raccoons and zoo animals. However, in the past the virus hasn’t been known to spread easily from mammal to mammal; the infections documented have been from direct contact with infected birds. 

This changed very recently with an outbreak of avian flu on a Spanish mink farm in October of 2022 and another amongst sea lions in Peru in February of 2023. From these events, scientists concluded that mammal-to-mammal transmission was extremely likely. 

An 11-year-old girl in Cambodia was recently diagnosed with and eventually died of avian flu, and while 11 of her close contacts tested negative, her father also tested positive for the virus. Investigations are currently ongoing as to whether human-to-human transmission was involved, or if both infections were from direct contact with infected birds or animals. 

Omer stressed the importance of investigating clusters of mammalian outbreaks and conducting public health surveillance of avian flu, including genomic testing, following the evolution of the virus. In addition, Omer believes that the government should begin making preparations now in case a human outbreak of avian flu does occur. 

These measures would include having supplies in place to start manufacturing vaccines if needed and preparing to effectively communicate health risks with the public. He emphasized that starting preparations now will drastically reduce reaction time if larger public health measures are needed in the future. 

Particularly in the case of the Cambodia cluster, Omer emphasized that it is crucial to determine whether human-to-human transmission occurred, using methods of “shoe-leather epidemiology” — or investigating the outbreak on a local population level — when needed.

According to Omer, if it seems likely that human-to-human transmission has occurred in Cambodia, more work must be done on an epidemiological level. The next steps would be to create mathematical models to inform transmission dynamics in humans, determine the parameters for these models, examine avian flu for cross-immunity with other influenza viruses and figure out other specific transmission dynamics of H5N1 to inform public health experts as quickly as possible. 

However, Ko and Omer still both emphasized the importance of balancing preparation with panic: although public health officials should be preparing, at this time, the general public does not need to modify behaviors or take extra precautions against avian flu. 

“We still need to be vigilant, since this is spreading around the world [and] there’s an opportunity for spillover to humans as it is being transmitted and accumulating mutations,” Ko said. “I wouldn’t raise the warning sign that we’re going to have another [human] pandemic, but we have to be vigilant.”

The Yale School of Public Health is located at 60 College St. 

Jessica Kasamoto covers the Yale School of Public Health for the SciTech desk. She is a graduate student in computational biology and bioinformatics.