Receiving two vaccine doses against measles as a child may not always lead to lifelong immunity, despite what is commonly thought, according to a modelling study. Even so, this remains the best way to safeguard against the illness.
The level of protection seems to fall by a very small amount per year, which could explain why a growing number of people are catching measles despite having had two vaccines against it in childhood. However, the shots are still 97 per cent effective, even by the time people are 30.
The findings come from analysing the pattern of measles cases in recent years in the UK and comparing it with mathematical models where protection either slowly wanes over time or is lifelong.
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Measles is highly infectious. It usually causes a fever, widespread rash and cold-like symptoms, although occasionally complications can lead to severe impacts such as brain damage. There are other potential effects too. Because the virus infects immune cells, it also causes a weakening of the immune system against other infections, called immune amnesia, that lasts from months to years.
Measles cases had been declining worldwide with growing use of the MMR vaccines, which protect against measles, mumps and rubella, and are usually offered to children as two doses when they are 1 and 3 years old. But falling rates of take-up have led to a resurgence of measles in many countries, such as the UK.
Getting both MMR vaccines gives a high degree of protection from measles infection, thought to be over 99 per cent a few weeks after the second dose. But some people still catch the virus despite being double vaccinated.
There could be two explanations for this, says at the London School of Hygiene & Tropical Medicine. The vaccines could have been ineffective initially, something that happens more often in infants who still have antibodies to measles in their blood passed on during pregnancy, he says. Such antibodies would attack the weakened form of the virus used in the vaccine. Alternatively, the person鈥檚 immunity to the illness could have waned over time.
Some previous studies looking at blood samples have found that antibodies to measles do . But antibodies are only one part of the immune system, so those studies don鈥檛 reveal if people鈥檚 protection against infection is indeed declining.
To find out more, Robert鈥檚 team analysed all the cases of measles reported to Public Health England from 2010 to 2019, noting the person鈥檚 age and vaccination status.
Over the nine-year period,聽there was no clear upwards or downwards trend for the total number of measles cases in England. But a slowly growing proportion of those cases affected people who had received both MMR vaccines, reaching 7.5 per cent by 2019.
The team ran modelling software to simulate the pattern of cases over time. The models that best fit the real-life data assumed that immunity slowly wanes.
Models that assumed no waning led to far fewer cases in double vaccinated people than the number of cases in real life. 鈥淥nly models with waning of immunity could capture the number and age distribution of vaccinated individuals infected by measles,鈥� says Robert.
All the models suggested that waning of immunity was slow, however, with a drop of less than 0.1 per cent per year. 鈥淭his would mean that individuals at 30 who gained protection after vaccination would still be at least 97 per cent protected against infection,鈥� says Robert. 鈥淢easles is so infectious that even a slow waning will lead to transmission in some vaccinated individuals.鈥�
It is too soon to recommend a third MMR vaccination, as more studies are needed to estimate the impact on transmission, says Robert.
at Erasmus University Rotterdam in the Netherlands says the work is a novel way to investigate the question of immunity waning. 鈥淢odelling of infectious diseases can give you information that鈥檚 simply not possible for the human brain to collect from large datasets,鈥� he says.
Despite the suggestion of waning immunity, at Swansea University in the UK says the MMR vaccines are still 鈥渋ncredibly safe and effective鈥�. 鈥淭hat said, the scientific community are constantly reviewing evidence and if, in the future, evidence grows as to the waning immunity of measles vaccines over the longer term, then no doubt discussions will be had as to the costs and benefits of offering a booster dose at a later age,鈥� he says.
medRxiv
