An Update on Measles, Pertussis, Mpox, and Other Vaccine-Preventable Diseases
What’s behind the resurgence of diseases we know how to prevent?
During the COVID-19 pandemic, the U.S. saw dramatic reductions in infections spread by the respiratory route, including measles and pertussis. These two diseases are also vaccine-preventable, and the U.S. has high immunization rates for both of them. So why are measles and pertussis cases ticking up now?
In this Q&A, adapted from the November 13 episode of Public Health On Call, Lindsay Smith Rogers speaks with vaccine expert William Moss, MD, MPH, about the reasons for these outbreaks. Moss, executive director of the International Vaccine Access Center and professor in Epidemiology, also discusses global outbreaks of mpox and the vaccines available to prevent it, as well as vaccines in the pipeline for malaria and tuberculosis.
We’re seeing outbreaks in the U.S. of vaccine-preventable diseases. What are the big stories here?
We’ve done remarkably well in preventing vaccine-preventable diseases and in achieving high immunization coverage in the United States. But there are communities throughout the U.S. where vaccine coverage is lower than it needs to be.
For example, we are seeing an increasing number of measles cases this year in the U.S. compared to what we’ve seen over the past couple years. During the COVID pandemic, prevention measures like restricting travel and wearing masks had an impact on all respiratory pathogens, including measles, and we saw historically low numbers of cases. In 2019, right before the pandemic, there was a global resurgence of measles, and we saw 1,274 cases in the U.S.—more than we had seen since 1992. This year, we have 262 cases so far.
Are you seeing anything unusual about these cases, other than the number?
One interesting thing is that these cases are across all age groups. About a third are in people ages 5 to 19; more than a quarter are in people older than 20. Another striking thing is that 40% of people with measles in the U.S. this year have been hospitalized—some for complications, some for isolation—and a lot of those are adolescents and older adults.
There are on the order of 100,000 to 200,000 measles deaths globally each year, and most of those are among young children. But measles can also be a severe disease in older individuals. We talk about this as a J-shaped curve, with high morbidity and mortality in both the very young and older individuals. It’s very important that we protect not only infants and young children from measles, but also older adults who are at higher risk of complications.
But measles is vaccine-preventable.
Yes, and not surprisingly, most cases are in unvaccinated people. We do see a small proportion of cases in the most recent CDC statistics—8% of individuals the U.S. with confirmed measles—who received one dose of a measles vaccine. Four percent had received the recommended two doses, at least by history of record. Those are what we call breakthrough cases: individuals who are fully vaccinated but still get measles. But 88% of the cases in the U.S. were either unvaccinated or had an unknown vaccination status.
Should people be asking their doctors to test whether they have immunity to measles?
Immunity from the measles vaccine lasts a very long time. There’s some waning of immunity over time, but we don’t recommend that the general public have their antibodies checked. Some people, such as people working in hospitals or health care facilities, do get tested. But in general, we assume that the protection from two doses of measles vaccines lasts a lifetime.
Another preventable disease started to creep up last year: pertussis. Can you tell us about that?
Pertussis, also known as whooping cough, is caused by the bacterium Bordetella pertussis and, like measles, is transmitted by the respiratory route. So again, during the early years of the pandemic, when we were doing a lot to prevent the transmission of SARS-CoV-2, we also saw a reduction in pertussis cases in the U.S. But this year, we’re seeing a marked increase from what we’ve seen over the past three or four years, about 15,000 cases here in the U.S. That’s even higher than what we saw in 2019 and about five times higher than what we saw last year. Again, it’s a vaccine-preventable disease.
We do tend to see periodic increases in pertussis cases in the U.S. One theory that explains these increases is that the U.S. switched in the late 1990s from what was called a whole-cell pertussis vaccine to an acellular pertussis vaccine. The whole-cell vaccine, which many countries still use, induces a very strong immune response, but it can cause a lot of inflammation and some related side effects. So in 1997, the CDC’s Advisory Committee on Immunization Practices recommended that we move to using the acellular pertussis vaccine, but the immunity from that vaccine doesn’t last as long as immunity from the whole-cell vaccine. There’s definite waning of immunity and increasing susceptibility of vaccinated individuals, and this is why booster doses can be helpful.
Do we know whether the people getting pertussis are unvaccinated or previously vaccinated?
The CDC doesn’t report those proportions specifically, but I would say it is a mix of individuals, and probably a small proportion of people who are unvaccinated. The coverage with the diphtheria, tetanus, and pertussis, or DTaP, vaccine here in the U.S. is very high among children, so I’m surmising that these are individuals who were previously vaccinated. Maybe they didn’t get the full five-dose course in childhood. But combined with waning immunity from the acellular pertussis vaccine, we’ll see a much higher proportion of vaccinated individuals getting pertussis than we saw with measles.
What are we seeing globally in outbreaks of vaccine-preventable diseases?
There have been global increases in cases of measles, and that’s what’s really driving the increase in the U.S.
We are also now seeing outbreaks of two different clades of mpox. The one that most people are probably familiar with was in 2022, when the WHO declared a public health emergency of international concern. That outbreak of the clade II virus spread throughout the U.S. and Europe; it was a global pandemic that largely impacted men who have sex with men.
This past August, the WHO declared a second public health emergency of international concern because of an outbreak of the clade I virus starting in the eastern Democratic Republic of the Congo. There have also been cases identified outside the African continent, for example, in Sweden. It is just a matter of time before we see this clade I of the mpox virus here in the U.S.
The vaccines we have worked against both of them. In the U.S., we have two FDA-approved vaccines: JYNNEOS and ACAM2000. The Department of Health and Human Services donated a million doses of the JYNNEOS vaccine to help with the international mpox response, because countries like the DRC and neighboring countries have not had a sufficient number of vaccine doses to protect their populations.
It seems that the problem is less with the supply of mpox vaccines than the distribution. Is that what you’ve been seeing?
Yes, and this is not an uncommon problem. What’s upsetting is that this is exactly what we saw with COVID-19 vaccines: Wealthier countries that could afford to set up bilateral arrangements with the vaccine manufacturers often had more vaccine stock than they could actually use, while lower-income countries, particularly countries in sub-Saharan Africa and Asia, did not have access to the vaccines. What we saw play out during the COVID pandemic, with the inequity in vaccine access, we’re seeing again with mpox. This is a serious, important problem that we need to address.
And it seems even more complicated, because this is not a one-and-done vaccine.
Exactly. The JYNNEOS vaccine is a two-dose schedule, and there’s ongoing discussion about whether there need to be booster doses because some recent studies suggest some waning immunity.
This challenge isn’t unique to mpox, but it’s made even more difficult because of the target population. We’ve gotten pretty good at getting vaccines to young children, but getting two doses or more into adults is challenging. And again, we saw that with COVID vaccines.
What’s in the pipeline for new vaccines?
First, some RSV vaccines came out over the past year or two—including one given to pregnant people in the last trimester. That’s an interesting vaccination strategy: to protect infants in the first few months of life with the passive transfer of protective antibodies from the mother to the infant in utero.
Some of the others I’m excited about are malaria vaccines. We now have two WHO prequalified vaccines for malaria, which kills close to half a million children each year. These vaccines, RTSs and R21/Matrix-M, are being rolled out in high-burden settings in sub-Saharan Africa. I’m very excited to see the continued rollout of these vaccines, but there are enormous challenges in getting them to children because they require multiple doses at an age range outside those in the traditional routine immunization program.
Also in the pipeline are vaccines for tuberculosis, which kills more people annually than any other pathogen. I think we’re on the brink of seeing effective vaccines against tuberculosis, and those will have a tremendous global public health impact.
This interview was edited for length and clarity.
