New projections warn that measles could once again become a permanent fixture in the U.S. unless vaccination rates improve, highlighting the urgent need to bolster immunization efforts amid rising vaccine hesitancy.
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In a recent study published in the JAMA (The Journal of the American Medical Association), researchers utilized nearly 20 years (2004-2023) of data to simulate 25-year trends in infectious disease incidence, hospitalizations, and deaths. Against the backdrop of declining childhood vaccination rates and ongoing debates about reducing childhood vaccination schedules, their models made several alarming predictions.
Firstly, if vaccination rates were to decrease by 50% of their current values, measles, rubella, poliomyelitis, and diphtheria cases would surge by a predicted mean of 51.2 million (95% Uncertainty Interval [UI], 49.7-52.5 million), 9.9 million (95% UI, 6.4-13.0 million), 4.3 million (95% UI, 4 cases to 21.5 million), and 197 (95% UI, 1-1000), respectively. This would result in an estimated 10.3 million (95% UI, 9.9-10.5 million) avoidable hospitalizations and 159,200 (95% UI, 151,200-164,700) deaths, along with tens of thousands of cases of severe complications like postmeasles neurological sequelae, congenital rubella syndrome, and paralytic poliomyelitis.
Secondly, even under current vaccination rates, measles is predicted to potentially stage a comeback and become endemic again in the US, occurring in 83% of the study's simulations, with a mean time of approximately 21 years (95% UI, 17.4-24.6 years). Together, these findings underscore the importance of childhood vaccinations and emphasize the need for increased vaccination coverage to prevent future outbreaks of vaccine-preventable diseases.
Background
Vaccines, biological preparations administered to help individuals develop immunity against specific infectious diseases, are a miracle of modern science and medicine. Since their discovery by Edward Jenner in 1796, vaccines have helped attenuate, restrict, and even eradicate several infectious diseases (e.g., smallpox), saving innumerable lives.
In the United States of America (US), childhood vaccination campaigns since the early 1900s have locally eliminated several infectious diseases, including measles, diphtheria, rubella, and poliomyelitis ('polio'). While sporadic infections do occur, continued vaccination has maintained population-level immunity above the threshold required to prevent these isolated incidences from causing epidemic-like outbreaks.
Unfortunately, the US has witnessed declining childhood vaccination rates over recent years, a condition exacerbated by the COVID-19 pandemic and its aftermath of vaccine distrust, misinformation, and other societal factors. While the US may be safe from vaccine-preventable infectious diseases for the moment, the future impacts of today’s declining vaccination coverage trends and increasing antivaccine sentiments remain unknown.
Recently, a larger US policy debate has begun regarding the revision of the long-standing childhood vaccination schedule, including the cessation of routine vaccine recommendations and the removal of school-based vaccine mandates for currently eliminated diseases, which would lead to substantially lower vaccine coverage.
About the study
The present study aims to inform the ongoing vaccination debate and prepare public health services for the outcomes of childhood vaccination reductions. It utilizes simulation models to elucidate the long-term impacts of various vaccination coverage scenarios over the next 25 years. The study focuses on four diseases that have been eliminated: measles, diphtheria, rubella, and polio.
Study data were obtained from the US Census Bureau (2019), which comprised state-specific birth rates, population sizes, and age distributions. Additionally, the National Center for Health Statistics was queried for death rates, and the National Immunization Surveys were used to obtain age-specific population immunity profiles (query years: 2004-2023).
Individual simulation models were developed for each disease under investigation, encapsulating its unique pathogen characteristics, including the basic reproduction number, infection potential, latent periods, and risk of complications. These models were run independently for each of the 50 US states and the District of Columbia. Historical infection estimates were used to parameterize infection importation (from other countries), assuming Poisson distributions. It's important to note that, like all modeling studies, this one involves assumptions and limitations, such as not modeling transmission between states or potential public health responses to outbreaks, which could affect the precise outcomes.
At the start of the simulation, each individual in the population was assigned to an age group and a state of residence, along with a pathogen-specific immunity status determined by their age and state, which dictated their initial compartment assignment. Susceptible individuals exposed to infection progressed through the model’s individual health states from susceptible to exposed, infectious, and then recovered for each pathogen.
Models were validated for accuracy and reliability by first simulating a five-year incidence period for each disease and comparing findings to present population-wide health records. Following validation, all models were coded to simulate different childhood vaccination rates, ranging from -100% to +10% compared to current vaccination rates (in 5% increments).
Study findings
Current vaccination coverage at the state level (from 2004-2023) was revealed to range from 88% to 96% for measles and rubella (via MMR vaccine), from 78% to 91% for diphtheria (via DTaP series), and from 90% to 97% for polio (via IPV series). These coverages correspond to an estimated childhood population-level immunity (ages 3-4 years) of 85-93% (measles and rubella), 78-91% (diphtheria, against severe disease), and 90-97% (polio, against paralytic disease), respectively.
Future projections using current vaccination coverage predict a mean of 851,300 cases (95% UI, 381,300-1.3 million) of measles, 190 (95% UI, 154-230) of rubella, 8 (95% UI, 1-22) of diphtheria, and 18 (95% UI, 3-61) of polio, corresponding to an estimated mean of 170,200 hospitalizations (95% UI, 76,200-250,000) and 2,550 deaths (95% UI, 1,130-3,760) over a 25-year period.
Alarmingly, these projections predict that even current vaccination coverage may be insufficient to prevent measles from reemerging as an endemic infectious disease in 83% of simulations, with a mean duration of approximately 21 years (95% UI, 17.4-24.6 years). However, a 5% increase in vaccination coverage was projected to substantially reduce measles cases to a mean of 5,800 (95% UI, 3,100-19,400), potentially preventing endemicity. An increase of 10% further reduced the predicted cases to 2,700 (95% UI, 2,200-3,400). In contrast, a 10% reduction in vaccination coverage from current levels would result in an estimated 11.1 million (95% UI, 10.1-12.1 million) measles cases.
Suppose current anti-vaccination sentiments result in a 50% reduction in vaccination coverage. In that scenario, the coming 25 years could witness an estimated 10.3 million (95% UI, 9.9-10.5 million) disease-related hospitalizations and 159,200 (95% UI, 151,200-164,700) deaths. This scenario also predicted severe complications, including 51,200 cases (95% UI, 49,600-52,600) of postmeasles neurological sequelae, 10,700 cases (95% UI, 6,700-14,600) of congenital rubella syndrome, and 5,400 cases (95% UI, 0-26,500) of paralytic poliomyelitis. Under this 50% decline, all evaluated pathogens except diphtheria were predicted to potentially establish endemic transmission (measles in >99% of simulations at a mean of 4.9 years; rubella in 100% of simulations at a mean of 18.1 years; polio in 56% of simulations at a mean of 19.6 years; diphtheria in <1% of simulations).
The study also found large variation in risk across different US states, with the paper specifically noting that the population in Texas was at the highest risk for measles.
Conclusions
The present study emphasizes the significance of childhood vaccinations in preventing the resurgence of several infectious diseases, potentially saving thousands of American lives and millions of dollars in medical expenses. It cautions against ongoing reductions in vaccination coverage, noting that, based on the model's assumptions (including constant vaccination rates and no reactive public health measures), even current coverage may be insufficient to prevent the re-emergence of measles as an endemic disease in the coming decades, and calls for an escalation in vaccination efforts.
While this summary includes significant technical details, such as uncertainty intervals, to reflect the study's findings and inherent uncertainties accurately, readers should consider these findings in conjunction with the study's stated modeling limitations and assumptions.
Declining childhood vaccination rates will increase the frequency and size of outbreaks of previously eliminated vaccine-preventable infections, ultimately leading to a resurgence of these infections to endemic levels. The timing and critical threshold for returning to endemicity will differ substantially by disease, with measles likely to be the first to return to endemic levels. This may occur even under current vaccination levels, without improved vaccine coverage and a robust public health response.
Article Revisions
- Apr 28 2025 - Correction to journal name - JAMA Network to JAMA (The Journal of the American Medical Association).