While a California law banning new personal belief exemptions (PBEs) for school vaccination requirements may reduce the size of measles outbreaks, the growing number of medical exemptions means large outbreaks are still possible, according to a model. predictive.
In a scenario looking at the time after California passed Senate Bill 277 (SB277) in 2015, the prevalence of PBE was reduced by 51.7%, but medical exemptions more than doubled, Ashley reported. Gromis, PhD, and Ka-Yuet Liu, DPhil, from the University of California, Los Angeles.
Additionally, the regrouping of these vaccine-exempt children resulted in much larger predicted measles outbreaks in California, they noted in Pediatrics.
From December 2014 to February 2015, California has experienced a measles outbreak out of 125 cases resulting from an exhibition in a Disney theme park. As a result, California passed SB277 in 2015, which eliminated the new PBE petitions, but did not revoke the old ones. In 2016, the state has seen medical exemptions for school immunization requirements increase dramatically.
“Our results (…) showed that the increased potential for large-scale epidemics persisted even after SB277 removed access to new non-medical exemptions,” Gromis and Liu wrote.
“In California, the lack of clear guidance on qualifying medical conditions, the authority to grant medical exemptions, and the institutions, if any, responsible for overseeing medical exemptions has likely contributed to the uneven increase in those exemptions to across the state, ”they added.
They also emphasized a measles epidemic in New York from September 2018 to July 2019, in which unvaccinated children resided in close proximity to each other and 649 cases of measles were confirmed. As a result of this and other outbreaks in New York State, the United States almost lost its measles elimination status in 2019, which she has owned since 2000.
“The policy implications of clustered vaccine-exempt populations require further consideration of alternative pathways for vaccine exemptions, including medical exemptions, especially in areas where vaccination rates are already low due to widespread reluctance. to immunization or barriers to accessing vaccines, ”wrote Yvonne Maldonado, MD, of Stanford University School of Medicine in Palo Alto, Calif., and colleagues from an accompanying commentary.
In the study, Gromis and Liu modeled the measles outbreaks in California under different vaccine exemption scenarios. In their model, the efficacy of the measles, mumps, and rubella (MMR) vaccine was set at 96.9% based on previous research.
The first scenario represented the 2014-2015 school year, before the adoption of SB277. The first seeded infection did not create any additional infections in 62% of 1000 trials.
In the second scenario, reflecting California post-SB277, this proportion increased to 66.1% of trials, but this difference was only marginally statistically significant (P= 0.056).
The most significant epidemics were observed before SB277, with maximum epidemics of over 4,000 infections. After SB277, peak epidemics were smaller but still significant, with over 1,000 infections. In this second scenario, the elimination of new PBEs reduced the average epidemic size by 86.5% and the maximum epidemic size by 70.8%.
“Gromis and Liu’s study reminds us of the vulnerabilities created by clusters of vaccine exemptions to highly transmissible viruses, and the risk of returning to a dangerous era where vaccine-preventable diseases may become endemic again,” Maldonado and his colleagues wrote.
The model represented all California children aged 0 to 17 in 2014. Demographic and census data were used to create a representative block-level population. In both scenarios, the prevalence data of vaccine exemptions by school and by daycare were used to recreate the spatial distributions of vaccine exemptions. Siblings were more likely to share PBE status in the first scenario and PBE and medical exemption status in the second.
When the vaccine-exempt children were randomized instead of grouped together in space in both scenarios, peak epidemics did not reach more than 26 cases, Gromis and Liu said.
They acknowledged that their study simulates the possible consequences of vaccine exemptions but does not measure the definitive risk. They also noted that they determined MMR vaccination status based on exempt status, not actual MMR vaccination data, although previous studies showed that MMR vaccination coverage in children benefiting from exemptions is low.
Interventions to increase immunization uptake “are relevant not only for routine school vaccinations, but may also become important as COVID-19 vaccinations are introduced for younger age groups , a potential reality in the coming months, “said Maldonado and his colleagues.
The writers and columnists did not report any disclosures.