Almost as soon as the hantavirus outbreak on the cruise ship MV Hondius became international news, public health experts rushed to assuage the public: This is not Covid-19. Don’t worry — this is a virus that requires “close contact” to spread. The risk of a pandemic is quite low.

But if this apparent certainty from public health leaders amid another emergent health crisis made you recall the earliest days of the Covid-19 emergency, you’re not alone. In February and March of 2020, health authorities also reassured the public that the novel coronavirus was not airborne (not true) and said that people shouldn’t wear masks (whoops). One of the central lessons of the pandemic was that health experts should not be overly confident in their public pronouncements, because any later changes based on new information could lead the public to lose trust.

So six years later, with hantavirus, you might wonder why health leaders sound so confident about controlling a virus with a much higher mortality rate than influenza or Covid-19. Wouldn’t it be better to be too aggressive rather than too cautious? One international group of doctors and scientists wrote an open letter on Substack to the World Health Organization, urging them to adopt a precaution-first approach. If there is any chance the hantavirus could be airborne or transmit more easily than “close contact” would suggest, public health authorities should assume the worst and act accordingly, the authors argued.

“The costs of implementing these protections early are modest,” they wrote. “The costs of delaying them during a high-consequence outbreak may be profound.”

Right now, the course of the hantavirus outbreak remains uncertain. There are still less than a dozen reported cases. How much bigger will it get? We don’t know for sure. A hantavirus outbreak has never occurred in this kind of environment before. Previous known outbreaks were small — a few dozen cases at most — and occurred in rural communities that are not conducive to widespread virus transmission. This one happened on a tightly packed cruise ship, with travelers from all over the world now returning to their home countries; it could be a perfect setting for rapid spread, as we know from the early days of Covid. How many of the passengers will get sick? Will they follow isolation protocols? How many more people will they infect? We don’t know.

But what’s already clear is that the shadow of the pandemic still hangs over both the public health organizations responsible for responding to the crisis and the increasingly distrustful public that those groups are supposed to serve.

Public health faces genuine dilemmas during any infectious disease emergency — how to balance the needs of the individual with the needs of the public at large, how to convey what they do (and don’t) know about a dangerous pathogen without confusing people if the situation changes. Public health authorities have struggled to strike the right tone in one of the highest-profile viral outbreaks since Covid-19.

“If you underreact, you can miss a window to contain something. But if you overreact without any clear evidence, you lose public trust, and you exhaust resources, and you make it harder to get compliance when you really need it,” Anne Rimoin, an epidemiologist at UCLA, told me. “I do think that public health officials are operating in a much more politicized, trust-fragile environment, and it certainly will make agencies more cautious about how they communicate uncertainty or escalate interventions.”

Hantavirus almost seems perfectly designed to poke the wounds of the pandemic. The superficial details of the cruise outbreak are too similar to the opening rounds of Covid to ignore: passengers forbidden from coming ashore, aggressive respiratory symptoms. And while scientists are undoubtedly more familiar with hantavirus, which has been documented and studied for more than 30 years, than they were with the virus that was eventually called SARS-Cov-2, there is still plenty we do not know about it.

Case studies from a 1993 outbreak in the southwestern United States and a 2018-19 outbreak in Argentina — the former the first documented cases of the deadly pulmonary syndrome caused by the strain of hantavirus common in the Americas, the latter a large and recent outbreak involving human-to-human transmission — are suggestive but hardly definitive. They both involved fewer than 40 cases and the evidence of hantavirus’s person-to-person transmissibility was mixed.

In Argentina, there appeared to be a superspreading event (a birthday party attended by the first symptomatic patient), that led to five new infections; one of those patients likely infected six more people. But there were also dozens of healthcare workers who treated infected patients, most without protective equipment, and none of them got sick. That led to the hypothesis that some people are more likely to spread the virus than others, but there isn’t a lot of clarity around which people it will be. The evidence suggests only that such spreaders have an unusually high viral load and compromised liver function during their infection.

And in both of those prior outbreaks, the virus took hold in sparsely populated rural areas. A cruise ship or a plane full of passengers is a very different environment. As Colleen Jonsson, a professor at the University of Tennessee Health Science Center and member of the International Society for Hantaviruses’s advisory board, put it to me, the rarity of hantavirus outbreaks means scientists are working with limited data as they attempt to predict what might happen next. “There are a lot of unknowns,” she said.

But at the same time, scientists have been studying hantavirus in animals and analyzing its genetics for decades now. In one bit of good news, a preliminary analysis of a sample from the cruise outbreak showed the virus was nearly 99 percent the same as a sample from the 2018-19 outbreak in Argentina and a 1997 outbreak there. One of the most challenging aspects of Covid-19 was how it mutated into new variants like delta and omicron that led to subsequent waves.

So based on what scientists know of hantavirus to date, this is not a virus that is very efficient at transmitting; there are hundreds of cases in North and South America every year. That’s why most public health experts, including everyone I’ve talked to, don’t believe hantavirus will cause a crisis comparable to Covid-19. (Though that is a very low bar.) But when you dive into the details, there is still a lot of uncertainty — and part of what the public health critics are focused on is what they see as a failure to convey those uncertainties and craft an outbreak response that accounts for them.

The worst-case scenario would be overly confident pronouncements about the relative lack of danger from hantavirus, only for poor infection control to lead to many more cases and deaths, further eroding whatever trust in public health authorities still remains. That is why many public health experts have been urging the WHO and its national partners to be more aggressive.

And in recent days, the WHO and its national partners have ramped up their response; for example, based on their ongoing investigations, WHO has upgraded their guidance to say everyone on the ship should be considered high-risk contacts. Most of the 18 American passengers who returned to the US are quarantined at the National Quarantine Unit in Nebraska and are expected to remain there for 42 days; two were sent to Atlanta for closer monitoring. (The incubation period for hantavirus can be as long as 40 days.) Other countries, including France, have also said they are requiring passengers to quarantine at a hospital.

Rimoin and Caitlin Rivers, senior scholar at the Johns Hopkins Center for Health Security, both said they thought that the response had progressed appropriately: based on the initial situation (infections among a husband and wife and a close contact) and prior history with the virus, it made sense to focus on close contacts. As evidence came in that even some people on the cruise who were not in close contact had gotten sick, WHO adjusted, deeming anyone on the ship as high-risk for infection.

“From the outside, that can sometimes look inconsistent. But in reality, it generally reflects the fact that outbreak investigations are iterative and data-driven,” Rimoin said. “It’s always easier to reconstruct the ideal response retrospectively once you know the outcome. But in real time, officials are making decisions with incomplete information. So far, we’re seeing the response escalate as the risk picture becomes clearer.”

In other words, some of this is a matter of perception. When I first reported on hantavirus last week, I was struck by the gap between the April 11 death of the first patient and the May 2 confirmation that hantavirus was the cause of the ship’s outbreak. How did it take three weeks to figure out what pathogen was spreading? But several of the experts I spoke to saw it differently: Rimoin said hantavirus “wouldn’t necessarily have been top of mind for anybody on that ship,” especially with respiratory symptoms. Jonsson was impressed that the South African facilities that accepted the infected passengers had the necessary diagnostic test on hand, and credited them for identifying the hantavirus as the cause as quickly as they did.

“All of those things are why it’s so complicated to be able to identify infectious diseases and to be able to react, because often by the time you realize what you have in front of you, you already have some spread,” Rimoin said.

Public health also requires risk management, balancing the need to protect the broader public with the rights and concerns of individuals. Rivers pointed out that keeping uninfected people on a cruise with infected people poses its own health risk to the first group.

But hantavirus is a reminder of how difficult that balance is to strike — and to communicate. And the challenge is only growing: We now have a general public traumatized and radicalized by the Covid-19 experience, something that was unprecedented in living memory. The mistakes made during that crisis conditioned people to be skeptical of WHO and public health authorities, whether they believe those experts are being too cautious, or not being aggressive enough.

“I think threading the needle is hard,” Rivers said. “And I know that there are some eerie parallels to Covid-19 that are putting people on edge.”

If the outbreak peters out, it will be vindication for the confidence that public health authorities projected and their efforts to manage the outbreak in a less draconian manner. But if it does spiral into a larger crisis, even if it falls short of something like Covid, it may be another breaking point for public trust in the WHO and public health at large.