Is bird flu a human epidemic? Here's what it would look like.

As far as public health officials know, the avian flu outbreak in dairy cows has so far only affected three farmworkers in the United States, all of whom have mostly mild symptoms.

But there's no guarantee that the virus, called H5N1, will remain harmless once it begins to spread among humans — in fact, accumulating evidence from the animal kingdom and data from other parts of the planet suggest the opposite.

Some dairy cows did not recover from H5N1, died or was massacred Infected terns became disoriented and could not fly. Elephant seal pups had difficulty breathing and suffered from tremors after contracting the virus. Infected cats I lost my sightwalking around in circles. Two thirds Among them Died.

“I don't think there's any room for complacency here,” said Annise Loewen, a virologist at Emory University.

“H5N1 is a highly pathogenic influenza virus and we should be of very high concern if it were to infect humans,” she said.

In ferrets ly inoculated with the virus through the eye – the presumed route of infection in US farm workers – the virus rapidly spread to the airways, lungs, stomach and brain, the team reports. report It was released on Wednesday.

Other studies have found similar patterns. Mice fed contaminated milkThe findings suggest that entry through the eyes or digestive system does not reduce the threat of the virus.

H5N1 has been shown to be promiscuous. Rapidly gaining new hosts It can also infect wild birds, poultry, rats, bears, cats, sea lions, etc. Since it was discovered in Hong Kong in 1996, about 900 people have been infected.

An older version of the virus circulating in Asia killed about half of those infected.

Of the 15 people infected with the strain currently circulating among cattle, one has died and another has been hospitalized in China, two patients have become seriously ill in Chile and Ecuador, and four people in the United States have become mildly ill – one infected last year and three infected during this current outbreak.

The point is, no strain of the bird flu virus appears to be able to transmit efficiently from person to person, and there's no guarantee that H5N1 won't acquire that ability, said Yoshihiro Kawaoka, a virologist and bird flu expert at the University of Wisconsin-Madison.

“We haven't seen any outbreaks in cows, so we think the nature of the virus is clearly changing,” Dr. Kawaoka said. The main symptom seen in two of the three farm workers, conjunctivitis, also known as pink eye, is atypical of H5N1 infection. The appearance of the virus in the mammary glands of cows and even in non-lactating mice was also unexpected.

The current concern is that as H5N1 continues to infect mammals and evolve, it could acquire the mutations necessary to spread efficiently among humans and spark a new pandemic.

The incubation period for influenza is two to four days, meaning person-to-person transmission can occur long before a case is detected, said Erin Sorrell, a virologist and senior scholar at the Johns Hopkins University Center for Health Security.

“Once it spreads to the general public, it's too late,” she said. “We've missed our chance.”

Influenza is most likely to cause serious illness in older people and children under the age of five. (The 2009 swine flu epidemic was not as devastating as feared, but it did cause serious illness in the elderly and children under the age of five.) killed The severity of the disease depends on how much virus an infected patient was exposed to and for how long, as well as the route of entry, genetic background, and overall health.

Infected people typically develop fever and respiratory symptoms, but in some cases can rapidly progress to pneumonia and death. If the avian flu virus were to adapt to humans, the world would need billions of doses of vaccines and antiviral drugs to prevent this.

There are four influenza antiviral drugs in the federal stockpile, but they must be taken within 48 hours of symptoms appearing to be effective. Recent Reviews For three of the four drugs, including the commonly used oseltamivir, sold as Tamiflu, there was found to be too little evidence to judge their effectiveness.

Among the new versions of H5N1, the virus Oseltamivir resistance and The other two drugsFortunately, however, these changes have not spread widely through animal populations, and no mutations have been observed to the fourth drug, baloxavir.

But there are only a few hundred thousand doses of that drug in the stockpile, according to David Boucher, infectious disease director at the federal Strategic Preparedness and Response Agency.

A vaccine would be a better option to stop a pandemic, but sufficient vaccines will not be available for at least several months. Even if global production of seasonal influenza vaccines were fully shifted to vaccines against H5N1, the amount of vaccines produced would be Less than 2 billion People assume two Dose Every single person needed it.

The U.S. has hundreds of thousands of vaccine doses stored in the national stockpile that can be distributed to at-risk people. Children includedDr. Boucher said companies under contract with the government could produce more than 100 million doses of the vaccine in the first 130 days.

Officials recently announced they have taken steps to prepare 4.8 million doses of the vaccine that can be bottled without disrupting seasonal flu vaccine production.

But most of these plans will only help if the virus cooperates.

H5N1 has branched out into many different forms since it first emerged, so scientists have created a library of 40 so-called candidate vaccine viruses to match it. Developing a new candidate can take up to three months, so having one ready to go can save valuable time, said Todd Davis, a virologist at the Centers for Disease Control and Prevention.

So far, the virus has remained largely unchanged, particularly the part of the virus that binds to human cells, called hemagglutinin (HA), he said.

Some experts say that if the virus is to spread among people, it would first have to change significantly. “If this virus infects humans, it would have to change its HA, because right now it doesn't bind very effectively to human cells,” said Scott Hensley, an immunologist at the University of Pennsylvania.

Conventional influenza vaccines inject candidate viruses into eggs or Mammalian cellsBoth have potential problems, such as the virus not replicating quickly enough, or mutating too much as it replicates.

In 2009, the candidate virus grew well in eggs but evolved into one that was incompatible with wild H5N1 viruses, leading to a long delay in distribution to the public. “By the time vaccine stocks were manufactured and distributed, the initial waves of the pandemic had already subsided,” Dr Hensley said.

CSL Sekisui, a leading manufacturer of seasonal influenza vaccines, Cell-based H5N1 vaccine It has already been approved by the Food and Drug Administration.

In the event of a pandemic, CSL executive director Mark Lacey said, if it had a vaccine candidate that matched the circulating strain, it could provide 150 million doses to the U.S. population within six months (the company also has agreements with 19 other countries).

But 150 million doses would only protect one in five Americans. Federal officials also said: explore As the COVID-19 pandemic has demonstrated, an mRNA avian influenza vaccine could potentially be produced very quickly to protect both cows and humans, and Dr Hensley's team is testing an mRNA vaccine in cows.

Authorities are hesitant to deploy Cattle Vaccines Experts said some countries have banned the import of products from vaccinated birds and animals due to trade concerns.

but Cattle immunization Experts said the move would limit the risk to farm workers and other cattle and limit the virus' opportunities to spread and evolve.

So far, federal authorities Vaccinating farm workersHe said the risk is still low.

Emory's Dr. Loewen said the real danger would be if farmworkers were infected with both H5N1 and seasonal influenza viruses: Influenza viruses are good at swapping genes, so co-infection would give H5N1 a chance to pick up genes and spread between people as efficiently as seasonal influenza.

This possibility highlights the importance of vaccinating agricultural workers, Dr. Loewen said: “Anything we can do to limit seasonal infections in people who are occupationally exposed to H5N1 can really reduce the risk.”