Last week, H5N1 bird flu infected two California dairy farm workers, bringing the total number of human cases in the U.S. to 16. These new cases are part of an ongoing outbreak that has remained minimal in humans but has spread more widely among livestock nationwide since March.

Meanwhile, a cluster of 8 suspected cases reported in Missouri has raised concerns about possible human-to-human transmission, but antibody testing for H5N1 in those affected is still underway.

Although this bird flu strain has been circulating for decades, the latest outbreak has made headlines because the virus is moving in a "worrisome direction," said Katelyn Jetelina, a prominent epidemiologist and data scientist. She noted that after the largest and longest outbreak in U.S. birds in 2022, the virus has jumped across species to infect mammals, including seals, mountain lions, foxes, and this year, dairy cows.

"This is significant because we have never seen this virus in dairy cows," Jetelina said. "Because humans are so closely connected to dairy cows... it has more opportunity to jump and mutate."

Jetelina launched a newsletter called "Your Local Epidemiologist" in March 2020, aiming to "translate" public health science for the public during the early days of the COVID-19 pandemic. The newsletter's popularity has since surged, now reaching over 230,000 readers in more than 100 countries, and helped Jetelina make TIME's list of the "100 Most Influential People in Health" earlier this year.

Here, Jetelina discusses the current H5N1 outbreak, the public health response, and next steps for vaccination.

This interview has been edited for length and style.

PHARMAVOICE: The current spread of H5N1 is puzzling scientists in many ways, including its cross-species jumps. But the CDC maintains that the threat level to humans is low. When would it reach a more alarming tipping point?

KATELYN JETELINA:Many of us are not satisfied with any of the signals we are seeing right now. There is a lot of concern among immunologists and epidemiologists. Our alarm bells are ringing. But that doesn't mean the public's alarm bells have to ring as well.

Human-to-human transmission is the first domino to fall if this evolves into an epidemic or pandemic. So far, that has not happened. The news from Missouri is concerning, where there is a cluster of possible human cases, but we are waiting for the blood test results that are currently pending to confirm whether there is true human-to-human transmission. So we are all holding our breath and hoping for good luck. We don't know yet... they could just have COVID.

Mortality rate reports vary. Is there a consensus on this?

No. This is really difficult because you see the 50% case fatality rate cited, which is technically correct because that is the data listed on the WHO website. However, the "true" case fatality rate is likely lower for several reasons. First, 50% is based on detected cases. Past antibody studies show we are missing many documented infections, and there may even be asymptomatic infections. Second, when viruses mutate to adapt for human-to-human transmission, they have to make trade-offs. The trade-off a virus usually has to make is exchanging disease severity for transmissibility. So if it mutates to become transmissible among humans, it likely reduces disease severity. Third, the reason we don't know the true case fatality rate is that we may have some cross-protection from regular flu strains.

Whether it's 50% or 5%, we learned during COVID that even a small percentage of a large number is a lot of people. It could still be devastating.

In your view, how is the current public health response in the U.S.?

It is heartbreaking to watch. After the pandemic, we hoped that losing 1.5 million people would change things, but it seems not much has changed. There is still this initial slow response. There is confusion, frustration, and a lack of communication between institutional coordination—for example, between the USDA, which handles animals, and the CDC and HHS, which handle humans.

This is also starting to expose a fragmented public health system that we haven't truly fixed. The law clearly states that public health is local. All public health powers rest with the states. We can argue about whether that makes sense, but it is clear that when facing a cross-state emergency or response, it becomes extremely difficult to quickly access information and engage the public. The complexity of dealing with both animals and humans also adds difficulty, which did not exist during COVID. I wish we could be more coordinated, present a united front with a unified voice, and be proactive rather than reactive, which is often how we in public health tend to be.

What does preparing for the worst-case scenario mean?

At the national level, it means the FDA has started preparing vaccines and building a stockpile. The FDA will hold a very interesting meeting (October 10) to discuss when and how to roll out an H5N1 vaccine.

At the field level, I would like to see more testing, more visibility into the scale of the outbreak and how far it is spreading. We are essentially flying blind right now.

How good is our vaccine stockpile?

About 5 million doses have been produced in the stockpile, which was done in a proactive manner this summer, and that is good. The question is: when do we start vaccinating the population? Finland has already started vaccinating high-risk groups. Another question is: how effective are they? I have not seen any data showing their effectiveness against the current strain spreading in dairy cows. But it is better than nothing... so we are optimistic that it will at least help prevent severe disease.

I am very curious about the direction the FDA will take on vaccine recommendations. Because the fact is, we are not seeing severe disease in farm workers. Actually, they are just getting mild red eye. We have only one severe case, but she had serious comorbidities in Missouri. So, what is the purpose of vaccination? Not only that, but also the feasibility of vaccination, given how low trust is among high-risk groups.