MMR Vaccine Effectiveness Declines Over Time

A modelling study conducted in England reveals that the amount of protection against measles declines slightly over time, but overall the MMR vaccine is still the greatest line of defense.

Over 95% of people who receive the MMR vaccine are protected from the measles for the rest of their lives.

Nonetheless, the study indicates that a spike in measles infections among those who have received two doses of MMR is consistent with the vaccine’s effectiveness declining by 0.04% annually following vaccination.

The proportion of measles cases among people in England who were “double vaccinated,” meaning they had received two doses of the MMR vaccine, increased from 1.9% (20/1064) to 7.2% (57/790) between 2011 and 2019, according to UKHSA statistics. It was unclear why this increase had occurred.

Using mathematical modeling, researchers at the London School of Hygiene & Tropical Medicine (LSHTM) investigated whether the rise could be attributed to the small percentage of individuals who do not receive protection from MMR after two doses or to the possibility that waning protection from the vaccine could also be a factor.

The team’s results, which were published in The Lancet Public Health, demonstrate that although the vaccine is still very protective, the number and age distribution of recent measles cases in England among double-vaccinated individuals could be most accurately predicted by a mathematical model with a very gradual decline in protection from MMR.

Lead author Dr. Alexis Robert, Research Fellow in Infectious Disease Modelling at LSHTM, stated: “It’s important to note that low vaccination rates are by far the biggest risk factor for measles outbreaks, even though 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.”

“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.

“This 0.04% waning each year is relatively slow, but because measles is so infectious, over time, this would add up to a ‘gap’ in a population’s defences the virus can exploit, which may increase the duration and size of outbreaks.”

Following an MMR vaccination, more than 95% of people develop immunity. This indicates that a tiny percentage of vaccinated individuals may become infected following two doses due to a lack of an immunological response. As a result, during outbreaks, a limited percentage of measles cases are anticipated in vaccinated individuals.

The authors modeled three potential scenarios in order to determine the causes of the recent rise in the percentage of measles cases among individuals who had received two vaccinations in England:

Immunity to vaccines does not wane.

2. Waning of immunity, or loss of protection, starts to happen every year at age five because by then practically everyone who has been vaccinated has had two doses.

3. Those who received their vaccinations prior to 2000 have complete protection up to that year, when measles was thought to be eradicated in England; nevertheless, starting from age five, immunity began to decline annually. In this case, immunization alone usually confers protection in younger populations, as opposed to immunization with viral exposure.

Then, in order to determine which scenario most closely matched reality, the researchers compared each one to the actual confirmed cases of measles that occurred in England between 2010 and 2019. No modeling was carried out to predict outbreaks in the future.

Of the three simulated situations, the two that involved a decline in MMR vaccine immunity most closely resembled the actual distribution of measles cases among those who received two vaccinations, both in terms of age group and time. In both cases, vaccination effectiveness held steady for several decades but was thought to decline very gradually over time. The third scenario involves a reduction in vaccine effectiveness of about 0.04% annually, with declining immunity beginning when the measles is no longer endemic.

A decline of roughly 0.04% annually would indicate that, for a person born in 1995 who had two doses of the MMR vaccine before the age of five and gained full protection from the vaccine, vaccine effectiveness stays on average:

• 99.6% at fifteen years old

• 99.2% when 25 years old.

• 98.8% of those over 35

• 98.4% at 45 years old

The scientists accounted for the possibility of infection in the very tiny percentage of those who did not develop immunity after vaccination in all of the scenarios. The findings indicate that this was insufficient to account for the rise in cases of double vaccination that were seen in England, and the addition of a gradual decline in vaccine efficacy assisted in containing the measles outbreak.

The results could be partially explained by increased reporting and testing accessibility among highly vaccinated populations, which would lead to a rise in the number of measles cases among these groups. But because the increase was steady over time, seen in a variety of age groups and epidemics, and not limited to one, the findings point to a biological explanation, such diminishing immunity.

Even while the model only finds a slight decline in vaccine effectiveness with age, it shows that this immunity loss is significant enough to affect the percentage of vaccinated measles cases among sick individuals. Since persons infected by vaccinated instances may not have been infected without waning, the impact of waning is attributable to both the chance of an individual becoming infected and the risk of transmission from vaccinated cases. Vaccinated cases are only marginally less likely to transmit than unvaccinated cases, according to the model, which indicated that the risk of onward transmission from double-vaccinated individuals was 83% of the risk of onward transmission in unvaccinated people.

The best method to shield adults and children from measles outbreaks is for everyone to receive two doses of the MMR vaccine, the experts emphasize. Because measles is extremely contagious, experts claim that even a slight decrease in immunity might cause a “gap” in our defenses, which in turn can lead to measles outbreaks. Sustaining elevated vaccination rates would be necessary to reduce the potential for transmission resulting from a gradual decline in immunity.

While the WHO aim is 95%, the most recent UKHSA data reveal that just 85% of under-5 children in England have gotten two doses of the MMR vaccine.

“Our study looks at one small part of the picture of measles cases in England. By far the larger issue in terms of measles spread is that uptake of the MMR vaccine has been decreasing in England since 2015.

Understanding the impact of vaccine immunity waning will help anticipate the potential impact of measles in countries where incidence has been low for decades, but vaccine uptake is reducing. The best way to limit the impact of measles and protect everyone from what can be a horrible disease, is to keep vaccine uptake as high as possible.”- Dr. Anne Suffel, Co-Author from LSHTM

Dr. Adam Kucharski, Professor of Infectious Disease Epidemiology, co-author from LSHTM, said: “Our findings show that the measles dynamics observed in England are consistent with a slow waning of immunity in double-vaccinated individuals.

“Other factors may partly explain the increase in the proportion of vaccinated cases, such as changes in testing patterns over time. However, the consistency and age distribution of the increase in England – combined with reports of cases in vaccinated individuals in other countries and previous laboratory studies showing a decline in measles antibodies – suggests a biological explanation is involved.”

Dr Robert said: “It’s important to emphasise that the patterns we see in the data are only there because outbreaks have occurred as a result of declines in vaccine coverage. If there were no outbreaks, this small amount of waning would not show up in any data. The key issue here is coverage, not the effectiveness of the vaccine.”

For more information: Long-term waning of vaccine-induced immunity to measles in England: a mathematical modelling study, The Lancet Public Health, https://doi.org/10.1016/S2468-2667(24)00181-6