A new survey by the Annenberg Public Policy Center of the University of Pennsylvania finds a “small but significant decline” in the percentage of U.S. adults willing to recommend the measles, mumps, and rubella (MMR) vaccine—from 88% of respondents recommending that an eligible person in their household receive the vaccine in 2022 to 83% today.
This is an example of rising vaccine hesitancy due to a mix of fear, misinformation, and eroding trust in institutions, and it is dangerous even for vaccinated children because a small decrease in vaccination rates (such as the one suggested by the Annenberg survey) undermines herd immunity and allows outbreaks of highly contagious diseases like measles to spread. When too many children remain unvaccinated, even a small minority of vaccinated children who are not fully protected can get sick, and infants or immunocompromised classmates face life-threatening risks.
What “Vaccine Hesitancy” Means
Public health researchers define vaccine hesitancy as a delay in acceptance or refusal of vaccines despite the availability of vaccination services, often existing on a spectrum from uncertainty about specific shots to blanket rejection. Recent reviews indicate that hesitancy is increasing in pediatric settings, with more parents selectively delaying, spacing, or skipping routine vaccines for their children.
Vaccine hesitancy differs from access barriers: some families cannot get vaccines due to cost, logistics, or clinic access, whereas hesitant parents generally have access but lack confidence, feel complacent, or experience strong social and political pressures against vaccination. This distinction matters because solutions focused solely on access will not reverse attitudinal resistance or mistrust.
Key Drivers of Vaccine Hesitancy
Large systematic reviews and recent pediatric studies converge on several main drivers of vaccine hesitancy among parents: concerns about safety, doubts about effectiveness, misinformation and social media, distrust of institutions, and competing values like autonomy or “natural” health beliefs. Many hesitant parents do not see themselves as “anti-vaxers” but as cautious or selective, especially when new vaccines such as COVID-19 or RSV are introduced.
Common themes include fear of serious side effects, worries that there are “too many, too soon,” low perceived risk of the underlying diseases, and the belief that healthy children do not need vaccines. Social networks amplify these concerns; parents who have more vaccine-hesitant friends or online contacts are significantly less likely to vaccinate their children on schedule.
Role of Misinformation and Social Media
Negative vaccine content is highly visible online, and multiple studies link exposure to online misinformation with lower vaccination intention and greater hesitancy. False claims about autism, fertility problems, myocarditis risk, or “immune overload” continue to circulate, even though they have been extensively investigated and refuted in the scientific literature.
During the COVID-19 pandemic, political polarization and distrust of public health agencies deepened, and that mistrust has spilled over to other childhood vaccines, including measles, polio, and pertussis. Parents who mistrust government and public health authorities often turn to influencers, niche websites, or social media groups that reinforce hesitancy and conspiracy theories, creating echo chambers that are resistant to alternative perspectives.
Historically, strong recommendations from pediatricians were among the most powerful drivers of vaccine acceptance; however, qualitative work with mothers reveals a decline in trust in pediatricians and public health officials regarding newer vaccines. Some parents now say they respect their child’s doctor but prioritize advice from specific specialists, podcasts, or peers they perceive as more independent.
Broader societal trends like political polarization, historical inequities, and perceived conflicts of interest also fuel suspicions that vaccines are promoted for profit or control rather than health. When trust erodes, parents are more likely to interpret rare adverse events or routine post-vaccine discomfort as signals that the whole system is unsafe.
How Hesitancy Weakens Herd Immunity
Vaccines do not have to be 100% effective to control disease; they work at the population level by raising the percentage of people who are immune high enough that pathogens struggle to find new hosts, a concept known as herd immunity. For extremely contagious diseases like measles, models and historical data show that roughly 90–95% of people in a community must be immune to prevent sustained transmission.
When vaccine hesitancy pushes coverage below these thresholds, pockets of susceptibility develop where a single imported case can ignite an outbreak. These outbreaks often start and spread in communities or schools with clustered exemptions, illustrating how a minority of unvaccinated children can compromise protection for the whole group.
Why Vaccinated Children Are Still at Risk
No vaccine is perfect; for measles, two doses of the MMR vaccine are about 97% effective, and one dose is about 93% effective, meaning a small fraction of vaccinated children remain susceptible. In a highly vaccinated community, “breakthrough” cases are rare because there is little circulating virus; however, in under-vaccinated pockets experiencing outbreaks, that residual vulnerability becomes clinically significant.
During outbreaks, approximately 10% of reported measles cases in the United States occur in vaccinated individuals, which is expected given the high overall coverage but high viral exposure. Vaccinated children generally have milder disease and far lower risks of complications, but they can still get sick, miss school, and potentially transmit infection to infants too young to be vaccinated or to immunocompromised classmates.
Risks To Infants and Medically Vulnerable Children
Infants and immunocompromised children depend on the immunity of those around them because they either cannot be vaccinated yet or respond poorly to vaccines. Measles case-fatality rates are highest in children under one year, and severe complications such as pneumonia and encephalitis occur disproportionately in this age group.
Global data show that despite a safe and effective measles vaccine, approximately 95,000 people, mostly children under five, died from measles in 2024, and modeling suggests vaccination prevented an estimated 59 million deaths between 2000 and 2024. When local coverage falls, and outbreaks occur, these vulnerable children become the first and most severe casualties of what are otherwise preventable diseases.
Resurgence of Diseases Once Under Control
Declining vaccination coverage and rising hesitancy have contributed to the global resurgence of measles and other vaccine-preventable diseases. The World Health Organization and multiple reviews report sharp increases in measles cases since the COVID-19 pandemic, including large outbreaks in Europe and the United States, affecting mostly unvaccinated children.
In the U.S., recent measles outbreaks in states including Texas, Minnesota, and Utah have been linked to communities where school entry exemptions or delayed vaccination schedules have pushed local coverage well below the herd immunity threshold. These events illustrate how local decisions influenced by hesitancy can reverse decades of public health progress in a matter of seasons.
Recent South Carolina Example
In mid-December, the South Carolina Department of Public Health (DPH) responded to an accelerating measles outbreak in the Upstate region, primarily centered in Spartanburg County. The status of this outbreak, as of December 12th, was as follows:
- Total Cases: 129 total measles cases were reported in South Carolina in 2025, with 126 related to the Upstate outbreak that began in October.
- Recent Increase: The DPH reported 15 new cases since the preceding Tuesday alone. [Cases continued to rise to 153 in the Upstate region out of 156 total in South Carolina as of 12/23, suggesting that the outbreak will continue to spread.]
- Quarantines: 303 people were in quarantine, and 13 were in isolation to prevent further spread.
- Transmission: Most new cases stemmed from known household or neighborhood exposures, but cases with an unknown source suggested community transmission was ongoing.
- Impacted Schools: Public exposures were identified at 11 schools, with many students asked to quarantine.
The measles outbreak in South Carolina was caused by a combination of low vaccination rates in specific communities and the highly contagious nature of the virus. Key factors included:
- Low Vaccination Rates: The Upstate region, particularly Spartanburg County, has seen a dramatic increase in religious exemptions for the MMR (measles, mumps, and rubella) vaccine in schools over the past decade. This has resulted in vaccination rates below the 95% threshold required for herd immunity, allowing the disease to spread easily once introduced.
- Unvaccinated Individuals: The overwhelming majority of individuals infected in the current outbreak (119 out of 126 cases) were unvaccinated. This highlights that those without immunity are the most vulnerable and critical to the virus’s spread.
- Initial Introduction: The initial cases of the year were travel-related, but the large Upstate outbreak began with cases that had no identified travel history, suggesting community transmission was already occurring in a vulnerable population.
- Community Gatherings: The virus spread through various public exposure locations including schools (11 impacted), a church, households, and even a healthcare setting. The holiday season with increased social gatherings likely contributed to the acceleration of cases.
- High Contagiousness: Measles is an extremely contagious airborne virus, where up to 90% of exposed unvaccinated people will become infected. The virus can also linger in the air for up to two hours after an infected person has left a room, making containment difficult in under-vaccinated areas.
In essence, the decline in local vaccination coverage created a susceptible environment where imported or unidentified cases could quickly ignite an accelerating outbreak.
Why This Matters Even To Pro-Vaccine Parents
From a game-theory perspective, when most other parents vaccinate, some families may try to “free-ride” on herd immunity, accepting the benefit of community protection without accepting even small vaccine risks. But when enough families do this, the collective shield collapses: outbreaks emerge, and both vaccinated and unvaccinated children face higher exposure to dangerous pathogens.
For parents who vaccinate on schedule, rising hesitancy means increased risk that their children will attend schools, daycares, or camps with under-vaccinated peers, higher likelihood of quarantine or school closures during outbreaks, and greater danger to siblings or relatives who are infants or immunocompromised. In this sense, vaccine hesitancy is not just a personal choice but a population-level threat that erodes the safety net even for families committed to vaccination.