Interviews conducted by Reuters with more than 12 leading disease experts have shown that scientists tracking the spread of bird flu are increasingly concerned that gaps in surveillance could leave them several steps behind a new pandemic.

Many of them have been monitoring the new subtype of bird flu H5N1 in migratory birds since 2020. However, the spread of the virus to 129 dairy cattle herds in 12 U.S. states indicates a shift that could bring it closer to becoming transmissible between humans. Infections have also been found in other mammals, from alpacas to domestic cats.

Scott Hensley, a microbiology professor at the University of Pennsylvania, said, "It almost feels like a pandemic unfolding at a slow pace. Right now, the threat is very low... but that can change in the blink of an eye."

The earlier the warning about transmission to humans, the more global health officials can take steps to protect people by launching vaccine development, extensive testing, and containment measures.

Current federal surveillance of U.S. dairy cattle is limited to testing herds before they cross state lines. Government health officials and pandemic flu experts told Reuters that state testing efforts are inconsistent, while testing of people exposed to sick cattle is minimal.

Ron Fouchier, a Dutch flu virologist at the Erasmus Medical Center in Rotterdam, said, "You need to know which farms are positive, how many positive cows there are, how widespread the virus is, how long these cows remain infectious, and what the exact route of transmission is."

Dr. Jane M. P. Marrazo, director of the U.S. National Institute of Allergy and Infectious Diseases, said monitoring of humans is "very limited."

Marrazo described the human flu surveillance network run by the U.S. Centers for Disease Control and Prevention as "actually a passive reporting mechanism and a passive display mechanism." She added that the U.S. Department of Agriculture is more active in testing cattle, but does not announce affected farms.

Many experts said that the different approaches taken by animal and human health agencies could hinder a faster response.

Gigi Gronvall, a biowarfare expert at the Johns Hopkins Center for Health Security, explained, "If you were designing the system from scratch, you would have one agency. This is not the only example where we face environmental or animal issues causing human problems."

A spokesman for the U.S. Department of Agriculture said the agency is working "around the clock" with the CDC and other partners on a "whole-of-government response," adding that ongoing research shows that "the U.S. food supply remains safe, and sick cows generally recover after a few weeks. The risks to human health remain low."

The CDC stated in its announcement: "The USDA and state and local health departments across the country have been preparing for the emergence of a new influenza virus for almost two decades and are continuously monitoring even the smallest changes in the virus."

"A warning sign"

Some pandemics, including COVID-19, arrive without warning. Hensley noted that in the last flu pandemic, caused by the H1N1 virus in 2009, the virus and its ancestors first spread among animals for several years, but increased surveillance could have helped health authorities prepare.

Three people in the United States have tested positive for the bird flu virus H5N1 since late March after exposure to cattle, and they exhibited mild symptoms. One person in Mexico was infected with a separate strain of H5 that had not previously been seen in humans, with no known exposure to animals. Other cases have been reported in India, China, and Australia, caused by different strains.

The World Health Organization says the risk of H5N1 virus to humans is low because there is no evidence of human-to-human transmission. Some tools are available if that changes, including limited amounts of existing H5N1 vaccine and antiviral medications such as Tamiflu.

Wenqing Zhang, head of the influenza department at the UN agency, stated that there are mechanisms to rapidly initiate the production of tests, treatments, and vaccines if necessary.

Other experts say there is enough concern to start preparing for potential human spread, although the impetus for action varies depending on the role one plays in the response, according to Richard Hatchett, CEO of the Coalition for Epidemic Preparedness Innovations (CEPI). His organization early on funded the development of a vaccine against the coronavirus and is now in talks with research partners regarding H5N1.

The Coalition for Epidemic Preparedness Innovations (CEPI) aims to create a library of candidate vaccines for potential pandemic pathogens. This would help manufacturers to start large-scale production and distribute doses as needed within 100 days of an outbreak.

Some countries are taking steps to protect people from the H5N1 virus. The U.S. and Europe are securing doses of "pre-pandemic" flu vaccine that can be used for at-risk groups, including farm or laboratory workers. Finland is expected to be the first country to vaccinate workers on fur and poultry farms, as well as those involved in animal health response.

Zhang of the WHO stated that scaling up vaccine access is also complex, noting that manufacturers of potential pandemic influenza vaccines create seasonal flu vaccines and cannot produce both at the same time.

Since most flu vaccines are made using virus grown in eggs, it may take up to six months to produce pandemic vaccines. The U.S. is in talks with Moderna to use its faster mRNA technology for pandemic flu vaccines.

All experts acknowledged the need to balance acting quickly to avert a threat and overreacting.

Wendy Barclay, a virologist at University College London who conducts bird flu research for the UK Health Security Agency, said, "We want to raise our warning without saying the world is about to end."