Surprise! Relatively small decreases in vaccine uptake can lead to big increases in vaccine-preventable disease!

One of the most common tropes used by antivaxers is to attack herd immunity as not being real. Herd immunity, or as its sometimes called, community immunity, is a name for a phenomenon in which in a population with high levels of immunity to a disease members susceptible to the disease are protected. Basically, because the vast majority of members of the population are immune to a disease, that disease can’t gain a foothold in the population and lead to an outbreak or an epidemic. Basically, transmission from person to person is interrupted because any susceptible person who becomes infected is surrounded almost completely by people who are immune. Thus, although individuals or small groups of individuals can become infected, in a population where a sufficient percentage of is immune the immune population serves as a “firebreak” on outbreaks, preventing them from spreading too far, protecting both those who are vaccinated and those who are not.

In this day and age, herd immunity is almost always achieved not through natural infection, but rather through vaccination. It’s why public health officials emphasize high levels of vaccine uptake. How effective herd immunity is depends upon how infectious the agent being vaccinated against is, how effective the vaccine is, and what percentage of the population is protected by it. For instance, for a disease like measles, which is among the most contagious of vaccine-preventable diseases for which the vaccine is greater than 90% effective, the percentage of the population that needs to be vaccinated to achieve herd immunity typically from 90-95%.

So why would herd immunity be such a threat to antivaccine beliefs? The reason is simple. It’s another major benefit of vaccines. More importantly, the existence of herd immunity undercuts another favorite antivaccine talking point, which is that, by choosing not to vaccinate their children, antivaxers are not endangering any one and that vaccinated children have nothing to fear from their unvaccinated children, because they are protected. Of course, obviously one problem with that argument is that many vaccines are not 100% effective, which is another reason why why herd immunity is so important.

Sadly, we see evidence of this effectiveness when vaccine coverage falls. Antivaxers will frequently point out that vaccine coverage in a state is at a very high level, and that’s frequently true. However, it is pockets of low vaccine uptake, where vaccination rates fall below that needed for herd immunity in communities where antivaxers are concentrated. But what would happen if vaccine uptake declined nationwide? Well, a recently published study out of Stanford University asked that question. Basically, it was a study modeling what would happen under various scenarios if vaccine uptake. The reason is obvious:

The routine vaccination of children is declining in Texas and other areas of the United States where they allow for personal belief and other nonmedical exemptions to childhood vaccination requirements. In these areas, there is growing vaccine hesitancy—defined as a delay or refusal to accept vaccination based on personal beliefs despite availability— that could accelerate gaps in vaccine coverage across the United States. The determinants of the parental decisionmaking process on whether to vaccinate their child are complex and context specific, but are often influenced by misinformation, false claims regarding vaccine safety, and a low perceived risk of infectious diseases among other factors. While the sources driving vaccine hesitancy (eg, the “antivaxxer” movement, celebrity endorsement, and online content) have historically been outside of science and government, there have recently been calls for a special government commission on vaccine safety, despite overwhelming scientific consensus on the safety and effectiveness of vaccinations. If the panel were to draft policies that relax childhood vaccinations requirements, the already declining trends in vaccination coverage in US children may decline further. The aim of this study was to estimate the potential public health and economic consequences of declining childhood vaccination, a result of a growing vaccine hesitancy movement, using the case example of measles, mumps, and rubella (MMR) vaccination and measles virus.

I’ve written before about declining vaccination uptake in Texas. It goes counter to the inaccurate stereotype of antivaccine beliefs and vaccine hesitancy as being primarily the province of hippy-dippy, granola-crunching lefties in that antivaccine sentiment is driven by a potent mix of pseudoscience and far right libertarian anti-government regulation sentiment. Basically, antivaxers have figured out how to weaponize their views by coupling them to right wing rhetoric about “freedom.” Not surprisingly, Dr. Peter Hotez, a pediatrician and public health researcher at Baylor and founding dean and chief of the Baylor College of Medicine National School of Tropical Medicine in the Department of pediatrics, is co-author on this study.

So what did the investigators do? Basically, they used publicly available data from the US Centers for Disease Control and Prevention to simulate county-level MMR vaccination coverage in children 2-11 years in the US. They applied a stochastic mathematical model, which was adapted to infectious disease transmission. Using the model, they estimated a distribution for outbreak size related to vaccine coverage. The predicted effects of declines in nonmedical exemptions were modeled and validated it against an independent data set from England and Wales.

So what did the model show? Not unexpectedly, with increasing nonmedical exemptions comes increasing numbers of measles cases:

The authors also found:

We found that a 5% decline in MMR vaccine coverage in US children would result in a 3-fold increase innational measles cases in this age group, for a total of 150 cases and an additional $2.1 million in economic costs to the public sector (Figure).With declining vaccination coverage, the size of outbreaks increased. Increased MMR vaccine coverage in children, through elimination of children with nonmedical exemptions or other mechanisms, increased national MMR coverage to95%prevalence (state variation, 91%-98%).We predicted that this strategy would reduce annual cases of measles by 20% (from 48 to 38 cases) and was an effective strategy to mitigate annual measles cases and costs.

Basically, minor declines in MMR vaccine coverage (in this case, from 93% to 88%) can result in major increases in the number of measles cases. Indeed, the authors note that their model, if anything, probably underestimates how much the number of measles cases can increase if MMR vaccine uptake falls significantly:

This modeling analysis likely predicts a conservative estimate for a rise in measles cases associated with declining immunization status because of foundational model assumptions and since we limited our analysis to US children (age 2-11 years). The model is designed to simulate outbreaks in highly immunized population and nonendemic settings. However, large reductions in MMR vaccine coverage could allow for measles to become endemic again, which is not accounted for in this model, and would likely result in thousands of annual measles cases. Owing to constraints on data for immunization status in the adult population and social mixing structures, we limited our analysis to children ages 2-11 years who contribute approximately 30% of the annual measles cases. However, the number of cases of measles would be much larger when accounting for other age groups, such as infants, adolescents, and adults. This increased number would be, in part, driven by infants younger than 12 months who are not yet eligible to receive measles vaccines, as illustrated in the sensitivity analysis modeling an expanded age group (0-11 years). Finally, the upper limit of the prediction interval in the base case analysis demonstrated substantial nonlinearity, suggesting the possibility for large outbreaks due to random chance with small reductions in vaccine coverage.

This is an important point. As is the case now, even with high overall vaccine uptake over a wide geographic area (e.g., the state of California or Texas) there can exist pockets of low vaccine uptake where outbreaks can and do occur. It is not unreasonable to predict that if the overall vaccine uptake for the whole country declines from 93% to 88% there will either be more areas with vaccine uptake low enough to compromise herd immunity or that existing areas of low uptake will have lower uptake still, to the point where it’s way below the level necessary for herd immunity. Then, of course, as the authors point out, the model doesn’t even account for all the vulnerable children, such as those too young to be vaccinated, as was pointed out:

“Outbreaks happen in communities, so we need to zoom in further than just national or statewide statistics when it comes to vaccination rates,” said Maimuna Majumder, a researcher at the Massachusetts Institute of Technology in Cambridge who wasn’t involved in the study.

For example, one recent study in California found county-level measles vaccination rates as low as 70 percent even though the statewide average was 90 percent, Majumder said by email.

Whenever a study or public health official has expressed concern about low vaccine uptake in California, as in during the debate in 2015 over SB 277, the bill that passed into law and eliminated non-medical exemptions to school vaccine mandates in California, antivaxers have keyed in on that one statistic, that statewide vaccine uptake was high. While high average uptake is a good thing, if there are pockets where the uptake is well below herd immunity levels, outbreaks of vaccine-preventable disease become more likely. Fortunately, SB 277 appears to have been working well thus far. In just one year, there are fewer counties with exemption rates high enough to be concerning, and exemption rates overall have plummeted. No wonder antivaxers hate it.

Obviously, this is a modeling study, and a model is only as good as the assumptions used to construction and the data used to generate it. However, the results of this study are in line with what we already know. Decreasing vaccine uptake leads to more outbreaks of vaccine-preventable diseases, more cases, and more suffering among children. (Just look at the Somali immigrant community in Minnesota, where American antivaxers have convinced the Somali community that the MMR vaccine causes autism, resulting in a huge plunge in MMR uptake and the expected concomitant measles outbreak.) Antivaxers will try to deny that, but the evidence is overwhelming that vaccines work and that when antivaccine sentiments take hold children are endangered. If we’re not careful, we could be sliding back towards a time when measles was endemic, as the UK did after Andrew Wakefield published (and publicized) fraudulent science that incorrectly concluded that vaccination with MMR was a risk factor for autism. The UK is only now recovering to the point that it was before Wakefield. If we’re not careful, we’ll be next.