UK researchers investigating a small rise in the number of measles infections in adults, despite two doses of MMR, have concluded the vaccine remains highly protective against the infection for life.

However, they found that on a population level, the effectiveness of the vaccine decreases by 0.04% each year after immunisation.

The analysis was done after figures showing the proportion of measles cases in doubly vaccinated adults in England tripled from 1.9% to 7.2% between 2011 and 2019.

A team from the London School of Hygiene and Tropical Medicine did mathematical modelling to find out whether a small proportion of people do not gain protection from MMR after two doses, or whether waning protection from the vaccine might also be a factor.

The analysis showed that a very gradual waning provided the best explanation for the figures, they reported in The Lancet Public Health.

But they said this small number of individuals had only been exposed to measles in the first place because of vaccine uptake had fallen below the threshold needed to stop outbreaks happening.

They explained that the picture had changed over time as more people had immunity from the MMR vaccine only rather than being exposed to measles infection.

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 would still be 98.4% on average at age 45.

Lead author Dr Alexis Robert, a mathematical modeller in infectious disease dynamics at LSHTM, said: ‘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.

‘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.’

He added: ‘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.’

The latest figures from the UK Health Security Agency show only 85% of under-fives in England have received two doses of MMR, well under the World Health Organization target of 95%.

Co-author Dr Anne Suffel, a researcher in the NIHR Health Protection Research Unit in Immunisation, said 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 Vanessa Saliba, consultant epidemiologist at the UKHSA, said: ‘This paper reinforces that MMR vaccine is highly effective in providing lifelong protection against measles.

‘In a highly vaccinated population, it is expected that we will see a very small number of measles cases in those fully vaccinated, but with milder symptoms and a much lower chance of passing the infection on to others.

‘The MMR vaccine remains the best and safest way to protect against measles and we encourage parents to check their children are up-to-date.’

A version of this article first appeared on our sister title, Pulse