This week we talk about polio, malaria, and smallpox.
We also discuss vaccines, the CDC, and the HHS.
Recommended Book: Remnant Population by Elizabeth Moon
Transcript
In 2000, the United States declared measles eliminated. That didn’t mean nobody in the country would ever catch it again, it meant the virus was no longer freely spreading within the US. New cases generally arrived with someone who had been infected elsewhere, and public health workers were able to stop those cases from turning into long-running chains of transmission via the systems, tools, and infrastructure they created.
In early October of 2026, Pennsylvania reported more than a thousand new measles cases this year, most of them part of an outbreak that began in April. As of October 7, the state had confirmed 1,078 cases across 41 counties, 207 hospitalizations, and five measles-associated deaths. Nationally, the Centers for Disease Control and Prevention, the CDC, had counted nearly 3,900 cases through the beginning of the month, making this the worst year for measles in the US in decades.
Pennsylvania’s latest update included 74 newly confirmed cases, though the state was careful to say that more than 40 of those people had become sick over two weeks earlier. So these were newly *reported* cases, not necessarily newly *infected* people—a distinction that matters when we’re trying to figure out whether an outbreak is speeding up or slowing down.
There’s a similar distinction being made with the death figures. Pennsylvania calls its five deaths *measles-associated*, using a definition that considers timing, a confirmed infection, medical evidence, and whether an unrelated cause might explain the death. The CDC’s national death count has not always matched the state’s, in part because the agencies publish on different schedules and review and tally deaths differently; so reporting on these sorts of things can sometimes be unintentionally fuzzy due to varying definitions and numbers.
What I’d like to talk about today is what it actually means to eliminate a disease, how measles has managed to claw its way back in a country that achieved that ‘eliminated’ milestone more than a quarter-century ago, and how vaccination, misinformation, and changing federal policy might shape what happens next.
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There are a few words public health workers and officials use that sound similar, but which describe substantially different achievements.
If a disease has been *eradicated*, it no longer circulates naturally anywhere in the world, as is the case with smallpox. The last known natural case was recorded in Somalia in 1977, and the World Health Organization declared the disease eradicated in 1980. There have been no natural smallpox outbreaks since. That’s a truly remarkable global achievement, and it’s why routine smallpox vaccination isn’t necessary for most people today.
*Elimination* is more local, and more conditional. It means sustained transmission of a disease has been interrupted in a defined place, like within a country. The pathogen can still exist elsewhere, and an infected traveler can still bring it in. Whether that introduction fizzles out or takes hold depends on what it finds when it arrives—how prepared that country’s government is, and how well vaccinated its population, how well funded its healthcare system, and so on.
Measles isn’t the only disease that’s been eliminated in the United States. The last US case of polio caused by *wild* poliovirus was in 1979, and the Americas were declared free of it in 1994. But in 2022, an unvaccinated person in New York developed paralytic polio caused by a vaccine-derived poliovirus, and genetically related virus residue was found in local wastewater. That was not wild polio somehow reappearing from the American past, it was a different type of poliovirus that latched onto a vulnerable person in a country where sustained polio transmission had long since been halted.
That virus was linked to the live oral polio vaccine used in some other countries, which can sometimes give rise to circulating variants where immunity is low. The inactivated polio vaccine used in the US cannot do that.
The US also eliminated sustained malaria transmission in the early 1950s, but mosquitoes capable of carrying the parasite still live in the US, and people returning from places where malaria is common can carry it across the border. In 2023, the CDC recorded ten locally acquired, mosquito-transmitted cases across four states—the first such cases in about twenty years. Ten cases doesn’t mean malaria has become endemic in the US again, it’s just further evidence that elimination of a disease in a region doesn’t mean reintroduction is impossible.
Smallpox, then, is an example of a disease that has been removed from natural circulation worldwide. Polio, malaria, and measles, in contrast, can be locally defeated, but regions in which that’s the case remain vulnerable to the movement of people, viruses or parasites, and that victory is reliant on the continued health of the health systems meant to detect and interrupt them.
Measles is an especially demanding test of those systems because it spreads so efficiently.
The virus moves through the air when an infected person breathes, coughs, or sneezes, and it can remain in the air, in a room in which someone has coughed, for up to two hours after that person has left. People can pass it on before the distinctive rash appears, and, according to the CDC, as many as nine out of ten nearby people who aren’t immune can become infected after exposure.
For some, the illness brings a fever, a cough, red eyes, and a rash, followed by recovery. For others, it causes pneumonia, swelling of the brain, or death. Young children, pregnant people, and those with weakened immune systems face higher risks, and this is why a measles outbreak isn’t just a rash going around, and why even just a few cases can generate an enormous amount of public health work: finding contacts, checking immunity, arranging vaccinations, and warning people who may have spent time in the same airspace. Measles can be painful and uncomfortable, but it can also be deadly, so all that effort is arguably well-invested.
The measles, mumps, and rubella vaccine—the MMR vaccine—is the main tool for preventing the spread of measles. One dose is about 93% effective against measles, and two are about 97% effective. That doesn’t mean vaccinated people can never get sick; these are estimates of how much the vaccine lowers the chance of infection. When most people in a community are protected, the virus also has fewer opportunities to reach those who cannot receive the vaccine or who haven’t yet reached the right age for their first routine dose.
Public health agencies often use about 95% coverage as a target for measles. That isn’t a magical national number, though, where a country is safe at 95.1% and doomed at 94.9%. It describes the level of protection usually needed within communities to make sustained spread difficult. A state can have a healthy-looking average while particular schools, neighborhoods, or social networks have far lower coverage and are thus at higher localized risk.
And that’s part of what makes the current trend concerning. CDC data show MMR coverage among American kindergarteners falling from 95.2% in the 2019-to-2020 school year to 92.4% in 2025-to-2026. About 280,000 kindergarteners lacked documentation of a completed MMR series in that latter year. Not all of them were necessarily unvaccinated (these sorts of records can be incomplete) but the direction of travel, and the concentration of missed vaccinations in some places and among some communities, leave more openings for the virus, which means everyone is at greater risk of infection.
There isn’t one explanation for all those missed doses. Some families have trouble getting appointments, taking time off work, or reaching a clinic. Others seek exemptions or have questions they don’t feel were answered well by medical professionals. Routine childhood care was interrupted during the pandemic, too. And misinformation has convinced some people that the risk from the vaccine is greater than the risk from measles itself, even though the evidence points strongly in the other direction.
Pennsylvania has some under-vaccinated communities, including in and around Lancaster County, which has been hit especially hard. But it would be misleading to treat one religious community, or one county, as the whole story here: cases have been recorded in 41 counties, and attitudes and access differ within any community. The point is that measles can exploit local gaps, then travel well beyond them.
One persistent claim is that the MMR vaccine causes autism. It doesn’t. This is a question researchers have studied extensively, including in a Danish study that followed more than 650,000 children and found no increased autism risk associated with MMR vaccination.
The claim has nevertheless remained influential, partly because it offers a simple explanation for something parents are understandably eager to understand. And the internet has made it easy for an old, discredited assertion to be repackaged as a newly discovered concern, often with a personal story attached, and a vibe of health and wellness that can conceal the fact that those making these claims are often famous or well-followed on social media, but generally ignorant about this subject. Those personal stories, though, can feel a lot more immediate and be a lot more memorable than statistics about a disease many Americans have never seen or experienced firsthand.
The current federal policy picture adds another layer of complexity. Robert F. Kennedy Jr., the Secretary of Health and Human Services, has a long history of raising doubts about vaccines, including the MMR shot. He has also, during the current outbreak, urged parents to vaccinate their children against measles; and that messaging muddlement confounds efforts by public health officials to get people in for regular vaccines, because not even the higher ups, the people in charge of this kind of thing at the federal level, are speaking clearly about it.
Speaking of those who are in charge of vaccines at the federal level: the Food and Drug Administration, the FDA, licenses vaccines after reviewing evidence of their safety and effectiveness. The CDC, advised by a committee of outside experts, makes recommendations about who should receive them and when. States generally decide what vaccinations schools require. The health secretary oversees the FDA and CDC, but changing a recommendation is not the same thing as withdrawing a vaccine’s approval, and neither automatically changes a state’s rules.
Under Kennedy, the FDA’s longtime top vaccine official, Peter Marks, left the agency in 2025 after a dispute with HHS leadership. The FDA also adopted a different, more risk-stratified framework for approving updated COVID vaccines. That change does not apply to the MMR vaccine, and it did not cause Pennsylvania’s outbreak, but it is part of a broader reconsideration of federal vaccine policy and the expertise used to set it.
At the CDC, Kennedy replaced the members of its vaccine advisory committee, and the administration pursued changes to the childhood immunization schedule. A federal judge paused some of those changes while litigation continues, so proposals and announcements should not be confused with settled, current guidance—a lot of this is still unsettled at this point, though RFK Jr’s efforts to reduce recommendations and approvals for vaccines have arguably already done a fair bit to make parents who were on the fence about getting these shots for their kids less likely to do so.
In August, President Trump signed an order calling for the combined MMR vaccine to be offered as three separate shots once those products become available, and for childhood vaccines to be spread across more visits where feasible. The administration presents that as a way to give parents more choice and build trust, and the order says the combined vaccine should remain available. But there are no single-antigen measles vaccines currently available in the US, and the CDC still recommends two doses of MMR. Pediatricians have warned that more appointments could mean more missed doses, especially for families already having trouble accessing care.
An executive order can’t itself manufacture those separate vaccines or license them; companies would have to produce them and the FDA would have to review them. All of which would take more time, and again, increase the complexity associated with trying to keep one’s child vaccinated.
Compared to the legal and political maneuverings related to vaccines, the underlying scientific questions are less unsettled. Extensive research has not found that the combined MMR shot causes autism, and two doses remain highly effective at preventing measles. It’s possible to investigate vaccine safety, improve transparency, and take parents’ concerns seriously without treating every claim circulating online, every random person casting doubt on vaccines, as equally supported by evidence; there’s no real comparison there, most of what’s being claimed by vaccine skeptics is unbacked bunk.
It’s also important not to draw a too-simple line from any one official’s actions to the current case count, though. The Pennsylvania outbreak began in April, and the children and adults most vulnerable to it often missed vaccinations years before this year’s executive order, and falling healthcare coverage predates the current administration. But today’s decisions can influence the information families receive, the ease with which one can get protected, and get one’s children protected, and the country’s ability to keep the next imported case from becoming the next large outbreak by ensuring the reality on the ground isn’t conducive to infection and transmission.
Pennsylvania has set up vaccination clinics in affected communities, and the state says healthcare providers gave more than 56,000 MMR doses in September, compared with about 25,000 in a typical month. New York declared a measles emergency in October and expanded the kinds of health professionals who can administer shots.
Pennsylvania also reported it has nearly 200 pop-up clinics in affected communities as of early October, alongside a marked increase in MMR doses given by healthcare providers statewide. Those efforts don’t erase the cases already recorded, but the hope is that they can keep disease spread limited.
The degree to which they manage that will be looked into by a Pan American Health Organization commission that’s scheduled to review the United States’ measles-elimination status in November. The question they’ll be asking is whether evidence shows the same lineage of measles has circulated continuously within the US for at least twelve months. Pennsylvania’s outbreak began too recently to answer that question by itself, so they’ll be looking into the bigger US picture, and that’ll determine whether the US keeps or loses its measles elimination status.
Canada lost its elimination status in 2025 after a year of continuous transmission. The United States may or may not receive the same classification; that determination depends on tracing and laboratory evidence, not on a single headline number in a given week. If a country does lose that status, though, it can regain it by once more interrupting sustained transmission; which usually isn’t easy, but it is doable—we’ve done it before, after all.
The larger lesson of measles, polio, and malaria is that eliminating a disease isn’t the same as getting rid of it for good. It’s more like maintaining a seawall: there may be long stretches when nothing appears to be happening, because the routine work is doing its job, keeping the tides at bay. The virus still exists on the other side of that barrier, though, and small gaps, tiny cracks can be all that virus needs to make it to the other side, and to start finding suitable hosts, again.
Show Notes
https://www.pa.gov/agencies/health/newsroom/pa-department-of-health-provides-statewide-update-on-measles-out9
https://www.pa.gov/agencies/health/diseases-conditions/infectious-disease/measles
https://www.cdc.gov/measles/data-research/index.html
https://www.cdc.gov/measles/about/index.html
https://www.cdc.gov/measles/causes/index.html
https://www.cdc.gov/measles/hcp/vaccine-considerations/index.html
https://www.paho.org/en/news/2-3-2026-update-review-measles-elimination-status
https://www.paho.org/en/news/10-11-2025-paho-calls-regional-action-americas-lose-measles-elimination-status
https://www.who.int/emergencies/situations/smallpox/
https://www.cdc.gov/mmwr/volumes/71/wr/mm7133e2.htm
https://www.cdc.gov/mmwr/volumes/75/rr/rr7501a1.htm
https://pubmed.ncbi.nlm.nih.gov/30831578/
https://transcripts.cnn.com/show/sotu/date/2026-08-02/segment/01
https://www.cdc.gov/measles/vaccines/index.html
https://www.fda.gov/news-events/press-announcements/statement-fda-commissioner-marty-makary-md-mph-100-days-embracing-gold-standard-science-transparency
https://apnews.com/article/7743be11cec4e4e22c50c2ddbcb6bcd8
https://apnews.com/article/aad16db9d014f9c024d8c42b2e2365bf
https://apnews.com/article/d10f81f221c4ae9f5b2f83dd0ee98b29
https://www.whitehouse.gov/presidential-actions/2026/08/delivering-gold-standard-childhood-vaccine-recommendations-for-americans/
https://apnews.com/article/586cedf31d3e51b501da196f0e5cbf2a
https://www.health.ny.gov/diseases/communicable/measles/
https://arstechnica.com/health/2026/10/pa-measles-outbreak-tops-1000-cases-largest-since-disease-was-eliminated/
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