UPDATED: The first potential US case of Andes hantavirus. Here are answers to the next questions.

May 14, 2026 Update: Today we were told that follow up testing on the individual who had a positive PCR for Andes hantavirus has since tested negative. Some now describe the initial test as “indeterminate.” HHS itself had posted that the test was “mildly PCR positive,” which to me implied that it was a confirmed (but likely not contagious) case, as explained below.

We’ll need more information before we know whether the individual had a false positive test or whether he was either at the end of his infection or that his immune system successfully defeated a very early infection that never fully took hold.

I’ll mention that the source of the information mattered here. The CDC is run by someone with a history of downplaying Covid-19. And HHS is run by Secretary Kennedy—who said he wants to give infectious diseases a break. So my interpretation of their social media post was that if even their minions had to admit that a test was “mildly PCR positive,” that the case was likely genuine. We’ll see if this person stays negative. I hope so!

May 12, 2026 Update: I’m updating this post now that I’ve gone through the guidance again after speaking to administration officials. I’m making changes to reflect that the CDC is technically not forcing anyone into quarantine—though as I will write in a new post, they kind of are without saying so directly, which is even more confusing.

This is an update on the Andes hantavirus outbreak. Overnight, the first US case of Andes hantavirus linked to the MV Hondius cruise ship was confirmed. Here’s what we know, what we don’t, and the questions that matter next. Thanks for being here, supporting independent reporting, and for helping spread reliable information!

CDC top official Dr. Jay Bhattacharya on CNN on Sunday. It did not go well, which was unexpected for a man who usually sounds smooth in interviews.

Last night, the Department of Health and Human Service announced that one of the US citizens en route home from the MV Hondius—the cruise ship with a cluster of Andes hantavirus cases—has tested “mildly positive” on a PCR test. Another US citizen en route has symptoms, but information about their test results was not provided. Separately, a French passenger has also tested positive.

Here’s the social media post from HHS (left) and my reply on Threads (right).

The rest of this post will be my best answers to what I assume are the natural next set of questions you may have, plus some analysis on public health decision-making. (Spoiler: It’s harder than our current officials realized back when they were private citizens complaining about Covid-19).

What does “tested mildly PCR positive” even mean?

First, if you are wondering what “tested mildly PCR positive” means, you’re not alone. As many have pointed out, saying someone tested “mildly positive” on a PCR for the Andes hantavirus is like being “sort of” pregnant. It’s a meaningless distinction. You either are positive or you are not. What HHS probably means is that there was a positive test with a high “cycle threshold” (for more on that, see below).

But the fact that HHS Secretary Robert F. Kennedy Jr said he planned to “give infectious diseases a break,” and that his NIH Director (who is also platooning as the CDC’s current top official), announced his intention to scrap plans that would prevent the next pandemic makes this clumsy verbiage look like the administration is not taking this threat seriously.

Thankfully, however, the actual professionals are. The patient is being transported to a special pathogens unit that is equipped to deal with situations like this.

Remind me what a “cycle threshold” is?

PCR tests work by amplifying genetic material. If there’s any viral genetic material in the sample, it will eventually be found by the test, replicated, and amplified. That process repeats and repeats until there’s enough genetic material in the tube for the test to turn positive.

So, it’s likely that “mildly positive” refers to a test that turned positive after many “cycles”—that is, the test was positive, but had a “high” cycle threshold.

Higher cycle thresholds mean lower viral loads.

Lower cycle thresholds mean higher viral loads.

Are they contagious?

We don’t know if the US patient is contagious. But positive tests with high cycle thresholds (i.e., lower viral loads) are far less contagious compared to positive tests with lower cycle thresholds. So that’s potentially good news.

But contagiousness is always a combination of biology and circumstance. So, the degree to which a patient is contagious depends on their viral load and the intensity of exposure to a potential next patient.

Why did I put potentially in italics above? Because when a test is positive, but has a high cycle threshold, it means one of three things:

1. Early infection. In this case, the virus may just now be reaching exponential growth. So, by the time the patient is retested, they might be extremely contagious.

2. An early infection that will clear. In some cases, the immune system is actively fending off a virus, and succeeding. But in the midst of the virus’s doomed struggle, it manages to have just high enough a viral load that the patient tests positive briefly. In these cases, the virus never reaches the exponential replication stage, and the infection is vanquished.

3. Late infection. In some cases, a positive test with a high cycle threshold may be the end of an infection. That is, their most contagious phase has passed, and things are improving.

Given the timeline of the MV Hondius Andes hantavirus cluster, I’m sorry to say that option 1 is by far the most likely. The good news is that this individual has been detected and will be properly isolated and cared for. Also, this virus has a longer interval between cases, which means its potential for rapid uncontrolled expansion—like we saw with Covid-19—is lower. The bad news, however, is that this likely means that more infections from the ship’s passengers can be expected. Getting this all squared away is going to take some time.

So, for HHS to choose the phrase “tested mildly positive on PCR” is likely both a reasonable rendering of how contagious the patient is (compared to other much more contagious cases), but is also downplaying the situation for that individual. Again, given the timing, the afflicted patient is unlikely to be the end of their infection. They’re likely to get sicker, and more contagious. That’s why it’s good that they are under excellent care already.

Could the test be a false positive?

All tests can be wrong. There can be false positives and false negatives. False positives are most likely when a condition is unlikely. For example, if I took 10,000 pregnancy tests, one of them might be faintly positive. But because I’m biologically unable to be pregnant, the most likely explanation for that positive test would simply be a bad reagent in the test kit.

So, could the American en route to US soil have had a false positive test for the Andes hantavirus? Yes, but that’s unlikely. He just disembarked from a ship with a known cluster of cases. This is what’s known as a “high pre-test probability” situation. In fact, you would be hard pressed to find me someone more likely to have acquired this virus than someone who just stepped off that exact boat. The likelihood that this was a false positive (known as a “posterior probability”) is quite low.

Will there be others?

Unfortunately, we can expect more cases from this ship. It seems like everyone who was on board has been tested, and may be tested many times in the coming weeks. Fortunately, these people know that they are high risk and this virus has slower dynamics than Covid-19 or influenza. That means that, despite another generation of cases (or even more), containment of this pathogen remains a highly realistic, and even likely outcome.

The only thing that I can think of that would change this would be if new mutations have made this virus’ life cycle dynamics much faster; or if asymptomatic spread or spread from casual contact is more routine than currently understood. (If this virus does spread in these ways, that has not been the common situation.)

Does this confirm human-to-human transmission occurred?

News of two new cases does not confirm that human-to-human transmission has occurred. It remains possible that an animal on the ship spread it to multiple people, or that the infected people visited the same place on land where the virus was waiting for them. But given that human-to-human superspreader events of the Andes hantavirus have been documented, I’d bet that human-to-human transmission did occur here. Remember that the doctor on the ship got infected. I think that greatly increases the chances that human-to-human transmission has occurred—because, otherwise, why him of all people?

What’s happening to the other passengers? And why is CDC lead Dr. Jay Bhattacharya suddenly Mr. Lockdown?

Each country is handling the repatriated cruise passengers and crew differently. The United States amended its guidance on Sunday, moving to two-tier system. Here’s the latest guidance for high- and low-risk contacts of Andes hantavirus patients.

High-Risk Contacts

Location and Housing

High-risk contacts have the option for home-based management (i.e., monitoring with modified activities) or facility-based management at the National Quarantine Unit in Nebraska or a location identified by a health department of jurisdiction.

If home-based management is preferred, the health department should coordinate with the high-risk contact to assess their capacity to follow public health directions. The individual should have a suitable home environment with access to a designated space in the home to isolate away from others immediately if symptoms develop, ideally with access to a private bathroom, for the duration of the monitoring period. Health departments should identify a hospital with capacity to isolate patients and to provide critical care, including extracorporeal membrane oxygenation (ECMO); CDC can provide technical assistance as needed.

Monitoring

Health departments should monitor high-risk contacts daily until 42 days after the last known high-risk exposure. For passengers

on the M/V Hondius, day 0 of the monitoring period is the date of disembarkation from the ship, provided no further exposures occur.

Travel

Health departments should advise high-risk contacts not to travel (domestically or internationally) during the monitoring period. However, if they do intend to travel, travel should be by chartered flight (no commercial air travel) and/or personal vehicle only.

Additionally, high-risk contacts should notify the health department of jurisdiction, and the health department should notify and coordinate with the receiving jurisdiction in advance of travel. If travel is international, CDC can assist with making notifications to destination authorities.

Activities

Health departments should advise high-risk contacts to modify their activities during the monitoring period to protect their household members and communities.

Low-Risk Contacts.

Health departments should regularly monitor low-risk contacts and advise them to self-monitor for fever and symptoms daily for 42 days after the last known exposure; there are no recommended travel restrictions or activity modifications other than general precautions such as hand hygiene.

If low-risk contacts intend to travel outside the jurisdiction, they should notify the health department of jurisdiction, and the health department should notify and coordinate with the receiving jurisdiction in advance of travel. If travel is international, CDC can assist with making notifications to destination authorities.

Source: CDC.gov

Analysis:

What counts as high versus low risk can be determined in the CDC guidance. However, I’ll note that among the features of high-risk contact, according to the CDC, is having been “within 6 feet of [an infected person] in an enclosed space for at least 15 minutes.”

Listen, this is obviously guesswork, and that’s fine. Public health officials have to implement policies based on their best judgment sometimes, including in the absence of ironclad data.

But it’s simply astonishing to see the 6 foot rule resurrected from a CDC now led by Dr. Jay Bhattacharya of all people—a man who was still so upset about the lack of science behind the 6 feet rule from the early days of the Covid-19 pandemic, that he still hadn’t let it go in the months prior to joining the Trump administration. Here’s a post of his on X from 2024:

Now that he's in the hot seat, suddenly 6 feet is good enough science to implement? Sure, why not?

Additionally, the CDC has also added a new high-risk feature to consider in its latest guidance: air travel. Now, if you were on an aircraft with a symptomatic Andes hantavirus patient and were sitting within two seats in any direction (or, maybe got too close whilst serving them a beverage, in the case of flight attendants), Dr. Bhattacharya will force you into lockdown for 42 days. wants you to stay home for 42 days, unless you need urgent medical attention. [Edit made at 7:30 p.m., May 12. Note: I’m writing a separate piece to describe the cognitive somersaults the CDC is doing here.]

Again, I’m not saying this is right or wrong. In fact, given the stakes, I’m sure quarantine for some or all high-risk persons is essential for public safety. But this policy is not based on anything like definitive science, and that irony can’t be ignored. The only reason Dr. Bhattacharya is in power is that he bellyached for years about precisely these types of decisions made during the outbreak of a novel pathogen (Covid-19) that, unlike this one, was spreading through our communities like wildfire.

My point is that, Gosh, it turns out that actually being responsible for things is really hard.

Can we test wastewater?

Now that there is at least one case of Andes hantavirus on US soil, an important question becomes how to track it. Can we use wastewater? Yes, apparently! I was told last night that commercial assays for testing Andes hantavirus already exist. Whether jurisdictions will want or need to start testing for the virus via wastewater is not yet known. But we can.

Who is in charge here?

As above, the CDC is currently being run by Dr. Jay Bhattacharya. The problem is that Dr. Bhattacharya is a health economist who did not do a medical residency, nor does he have much relevant public health experience here. Oh, and he’s also the confirmed Director of the National Institutes of Health. So, he’s spread a little thin.

To quote Dr. Bhattacharya: “For those of us who have dedicated our lives to public health, humility is not a weakness.”

Agreed. That’s why I think Jay should be humble enough to realize he cannot run two massive federal agencies at once—especially when one has been thrust into a complicated situation like the unfolding Andes hantavirus cluster.

If you think Dr. Bhattacharya is able to do both jobs, I invite you to watch this clip of him on CNN this weekend. He could not answer basic questions about the Andes hantavirus timeline. He also sounded uncharacteristically flustered. While I often disagree with his views, he’s usually quite smooth on the microphone. Here, he sounded out of his depth. It was actually kind of painful to watch. Meanwhile, the CDC lacks a Principal Deputy Director and other key positions.

We’re likely going to get out of this situation. But it won’t be because we’re flush with prepared leaders with adequate bandwidth, let me assure you.

The good news.

The reason we are likely to get through this situation is biology. We are fortunate that the Andes hantavirus does not appear to routinely spread asymptomatically, and that its serial interval—that is, the time needed for the virus to spread between people—seems to be long. One paper said the range is somewhere from 9 to 40 days. However, the CDC now says the range is 4 to 42 days.

I don’t love the change to 4 days. Hopefully those cases are outliers. Indeed, this virus’s generally slow biology means that even if there are leaks in the public health response and more cases occur, we can still expect to control this virus. Indeed, overall, viral dynamics favor containment.

But this isn’t over. What we do in the next days and weeks will determine how many people die, and how long this threat persists.

Resources:

• WHO Andes hantavirus outbreak page.

• Hantavirus Pulmonary Syndrome overview.

• Hemorrhagic fever with renal syndrome hantavirus.

• WHO facts sheet.

• JAMA patient information page.

• The Evidence Collective epidemiology terms.

Thank you for reading! If you have information about any of the unfolding stories we are following, please email me or find me on Signal at InsideMedicine.88.