The level of protection against measles provided by the MMR vaccine appears to decrease slightly in the long term.
If you were vaccinated against measles twice as a child, you will probably no longer be completely immune later in life. This follows from a new study. Yet vaccination remains the best way to protect yourself against the highly contagious disease.
The MMR vaccine protects against mumps, measles and rubella. It is offered to children in two doses, in the Netherlands and Belgium at the ages of one and nine.
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The research shows that the level of protection provided by this vaccine decreases slightly in the long term. This may explain why measles sometimes occurs in people who have been vaccinated against it twice as children.
Nevertheless, according to the study, the vaccinations are still more than 97 percent effective in thirty-year-olds. The study still needs to be verified by other scientists; a pre-publication can be found on the website medRxiv.
Revival
Measles is a highly contagious disease. It usually gives you a fever, widespread rash, and cold symptoms. Sometimes complications lead to more serious consequences such as brain damage.
The measles virus infects immune cells. This means that after an infection you are also less resistant to other infections. Such weakening of the immune system can last from months to years.
The number of measles cases has fallen worldwide in recent decades as more and more children received the MMR vaccine. But in recent years, a decline in vaccinations in many countries, including the Netherlands and Belgium, led to a resurgence of the disease.
Antibodies
If you have received both MMR vaccines, you are highly protected against a measles infection. A few weeks after the second dose, protection is estimated to be more than 99 percent. However, some people who have been vaccinated twice still contract the virus.
There are two possible explanations for this, says epidemiologist Alexis Robert of the London School of Hygiene & Tropical Medicine. First, the vaccines may have been ineffective in these people from the start. This mainly occurs in children with antibodies against measles in their blood that were passed on during pregnancy, says Robert. These antibodies can destroy the weakened form of the virus in the vaccine before it can do its work. The second possible explanation is that in vaccinated measles patients, immunity to the disease has decreased over time.
Some previous studies of blood samples have shown that the number of antibodies against measles decreases somewhat as people get older. But antibodies are only one part of the immune system. These studies do not show whether people are less protected against infection over time.
2010 to 2019
To find out more, Robert’s team analyzed all measles cases reported to Public Health England from 2010 to 2019. The researchers noted each patient’s age and vaccination status.
During that period there was no apparent increase or decrease in the overall number of measles cases in England. However, there was a gradual increase in cases among people who had received both MMR vaccines. In 2010, 1.2 percent of all measles patients had been vaccinated twice, in 2019 this was the case for 7.4 percent of patients.
The researchers used computer models to simulate how the effectiveness of the vaccine affects the number of cases over time. The models that best fit the real data were based on the assumption that immunity wanes slowly.
Third dose?
Models that assumed that immunity does not wane led to far fewer cases in double-vaccinated people than you actually see. “Only models of waning immunity could explain the proportion and age distribution of vaccinated individuals who became infected with measles,” says Robert.
According to the models, immunity decreases extremely slowly: the decrease is less than 0.1 percent per year. “That would mean that vaccinated people are still more than 97 percent protected against infection at the age of thirty,” says Robert. ‘But measles is so contagious that even a slow decline leads to transmission to some vaccinated people.’
More research is needed before a third dose of the MMR vaccine is possibly recommended, Robert says.
A new way
According to virologist Rik de Swart of Erasmus University Rotterdam, the research introduces a new way to study the issue of waning immunity. “Infectious disease modeling can give you information that the human brain simply cannot get from large data sets,” he says.
Despite the likely waning immunity, health scientist Simon Williams of Swansea University in the United Kingdom still calls the MMR vaccine “incredibly safe and effective.” ‘But the scientific community is constantly reviewing the evidence. “If there is more evidence in the future of the long-term waning immunity of measles vaccines, there will undoubtedly be discussions about the pros and cons of offering a booster dose later in life,” he says.
