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MMR Vaccine Most Effective Against Measles but May Wane Over Time: Lancet Public Health - Video

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Overview

Between 2011 and 2019, the proportion of measles cases in adults who received two doses of the MMR vaccine in England increased from 1.9% (20/1064) to 7.2% (57/790), however, the cause of this increase remains unclear.

A new retrospective modeling study, published in The Lancet Public Health journal, suggests slow MMR vaccine waning may partially contribute to rising measles cases in double-vaccinated people when outbreaks occur. However, the study authors emphasize that the MMR vaccine remains the most effective measure to prevent measles infection and that community vaccination levels above 95% are essential to prevent outbreaks in the first place.

In 2011 in England, 20 individuals who were double-vaccinated were infected with measles (1.9% of all cases) compared to 989 unvaccinated people (93% of all cases), whereas, in 2019 in England, 57 individuals vaccinated twice had measles (7.2% of all cases) compared to 666 unvaccinated people (84% of all cases).
More than 95% of individuals gain immunity after a single dose of MMR vaccine. This means that after two doses, a small proportion of vaccinated people (less than 1%) may be infected because of a lack of immune response. Due to this, it’s expected to see a small proportion of measles cases in vaccinated people.

This study is the first to use mathematical modeling to suggest measles cases and spread in England are consistent with a small amount of waning of vaccine-induced immunity.

“Although our results suggest that a slight waning of immunity from the MMR vaccine over time explains why we are seeing an increase in the proportion of measles cases in double-vaccinated people in England, it’s important to note that the biggest risk factor for measles outbreaks by far is low vaccination rates. The MMR vaccine remains highly effective and receiving two doses will protect you and those around you against measles infection. Even if you are one of the small number of people who get an infection after two doses of MMR vaccine, previous studies suggest measles symptoms in people who have been vaccinated are milder than in people who have not had a vaccine,” says Dr Alexis Robert, The London School of Hygiene & Tropical Medicine.
To assess the reasons behind the recent increase in proportion of measles cases in double-vaccinated people in England, the authors modelled three possible scenarios:
  • No vaccine waning immunity.
  • Waning of immunity increases each year from the age of five (as most vaccinated individuals have received their second dose by then).
  • Individuals vaccinated before 2000 have full protection until 2000 (when measles was considered no longer endemic in England) but waning of immunity increases each year from the age of five after 2000. In this scenario, immunity in younger groups generally comes only from vaccination, rather than from vaccination and exposure to the virus.
All scenarios included the risk of rare infections in double-vaccinated individuals who did not gain immunity when vaccinated. The researchers then compared each scenario with the real confirmed measles cases in England between 2010 and 2019 to see which scenario best fitted the reality. No modeling was done to forecast future outbreaks.
Out of the three modeled scenarios, the two scenarios containing waning immunity from the MMR vaccine best matched the real distribution of cases of measles in double-vaccinated people by age group and over time. In these two scenarios, vaccine effectiveness remained high after several decades but there was an estimated very slow decrease in effectiveness over time. In the third scenario, where waning immunity starts when measles is no longer endemic, vaccine effectiveness is reduced by approximately 0.04% per year.

People who have been vaccinated remain highly protected from measles. In the rare cases of a vaccinated person becoming infected with measles, previous research suggests their symptoms would likely be milder than people who have not received a vaccine, but they could still transmit the virus - in this study, the risk of onward transmission from rare cases of measles among vaccinated individuals was 83% that of unvaccinated individuals.

Reference: Funk, S., Knapp, J.K., Lebo, E. et al. Combining serological and contact data to derive target immunity levels for achieving and maintaining measles elimination. BMC Med 17, 180 (2019). https://doi.org/10.1186/s12916-019-1413-7

Speakers

Dr. Bhumika Maikhuri

BDS, MDS

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