Study shows gradual waning of MMR vaccine effectiveness over time

The MMR vaccine remains the best protection against measles, according to a modelling study in England which suggests the level of protection decreases slightly over time.

However, a rise in measles infections among people who have had two doses of MMR is in line with the effectiveness of the vaccine decreasing by 0.04% each year after vaccination, the study suggests.

Researchers at the London School of Hygiene & Tropical Medicine (LSHTM) used mathematical modelling to examine whether the rise could be explained by the small proportion of people who do not gain protection from MMR after two doses, or whether waning protection from the vaccine might also be a factor.

The team's findings, published in The Lancet Public Health, show that while the vaccine remains highly protective, a mathematical model with very gradual waning in protection from MMR could best reproduce the real number and age distribution of recent cases of measles in double-vaccinated people in England.

"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."

More than 95% of individuals gain immunity after a single dose of MMR vaccine. This means that after two doses, only a small proportion of vaccinated people may be infected because of a lack of immune response. Due to this, it is expected to see a small proportion of measles cases in vaccinated people during outbreaks.

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:

1. No vaccine waning immunity.

2. Waning of immunity (i.e. loss of protection) increases each year from the age of five (as almost all vaccinated individuals have received their second dose by then).

3. 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.

For someone born in 1995 who received two doses of the MMR vaccine before age five and gained full protection from the vaccine, a waning of about 0.04% each year would mean vaccine effectiveness remains on average:

• 99.2% at age 25

• 98.8% at age 35

• 98.4% at age 45

In all scenarios, the authors included a risk of infection in the very small proportion of individuals who did not gain immunity upon vaccination. The results show that this was not sufficient to explain the increase in double-vaccinated cases observed in England, and adding a slow decrease in vaccine effectiveness helped capture measles transmission.

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."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.

"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."

Source:

London School of Hygiene & Tropical Medicine (LSHTM)

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