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Most measles cases in the UK occur among unvaccinated people. However, between 2011 and 2019, the proportion of measles cases among adults who received two doses of MMR vaccine in the UK increased from 1.9% (20 of 1,064) to 7.2% (57 of 790). However, the cause of this increase is still unknown.
A new retrospective modeling study published in The Lancet Public Health suggests that the slow decline of the MMR vaccine may partially contribute to the increased number of measles cases in dually vaccinated people during outbreaks. Suggests. However, the study authors stressed that the MMR vaccine remains the most effective means of preventing measles infection and that community vaccination levels of >95% are essential to prevent outbreaks in the first place. There is.
Measles is a highly contagious disease that is spread by a virus that is easily transmitted when an infected person breathes, coughs, or sneezes. Although anyone can get measles, it most commonly occurs in children and can lead to serious illness, complications, and even death. Two doses of the MMR vaccine before age 5 is the best lifelong protection against measles.
Unlike other viruses, measles virus does not mutate much over time, so the effectiveness of childhood vaccinations remains high throughout life.
With increased coverage of measles vaccines, countries in Europe, the Americas, and Asia have observed significant reductions in the number of measles cases, with many years of no community transmission of measles and outbreaks occurring sporadically. However, a resurgence of measles cases was reported in these countries between 2015 and 2020, and in Europe the impact of the epidemic is increasing among young people.
Although the majority of cases occurred in areas with low vaccination coverage, the proportion of measles cases among dually vaccinated individuals is also increasing.
In the UK in 2011, 989 unvaccinated people (93% of all cases) became infected with measles, compared to 20 people who received two doses of the vaccine (1.9% of all cases). I did. Meanwhile, in the UK in 2019, 57 people infected with measles after receiving two doses of the vaccine (7.2%). This compared with 666 unvaccinated people (84% of all cases).
After one dose of the MMR vaccine, more than 95% of people become immune. This means that after two doses, a small proportion (less than 1%) of vaccinated people may become infected due to lack of immune response. Therefore, the incidence of measles in vaccinated persons is expected to be small.
However, it is unclear why this percentage has increased since 2010. There are two main explanations.
As the proportion of people who are immunized by vaccines rather than from higher-risk infections increases, the proportion of cases attributable to rare events in which there is a lack of immune response from both vaccinations is also expected to increase. Reduced immunity due to vaccines.
This study is the first to use mathematical modeling to suggest that measles cases and prevalence in the UK are consistent with a modest decline in vaccine-induced immunity.
“Our results show that a slight weakening of immunity from the MMR vaccine over time explains why the proportion of measles cases in the UK is increasing in people who have been vaccinated twice. “However, it is important to remember that the biggest risk factor for measles outbreaks is vaccination coverage, which is by far the lowest,” says Dr Alexis Robert, from the London School of Hygiene and Tropical Medicine.
“The MMR vaccine remains highly effective and two doses can protect you and those around you from measles infection. Even in one person, previous research suggests that symptoms of measles can be reduced with two doses of the vaccine.” Symptoms in vaccinated people are less severe than in people who are not vaccinated.” . ”
To assess the reasons for the recent increase in the proportion of measles cases among dually vaccinated people in the UK, the authors modeled three possible scenarios.
There are no vaccines that reduce immunity. Immunity decline increases each year starting at age 5 (as most vaccine recipients have received their second dose by then). People vaccinated before 2000 are fully protected until 2000 (measles was thought to be endemic in the UK), but after 2000 immunity starts to increase from the age of 5. The decline increases year by year. In this scenario, immunity for the younger group is generally obtained only through vaccination, or rather only through vaccination. than vaccination or exposure to the virus.
All scenarios included the risk of rare infections in dual-vaccinators who did not develop immunity at the time of vaccination. The researchers then compared each scenario to actual measles cases in the UK between 2010 and 2019 to see which scenario best matched reality. No modeling has been done to predict future outbreaks.
Of the three scenarios modeled, two involving waning immunity from the MMR vaccine best matched the actual distribution of measles cases by age group and over time in dually vaccinated individuals. In these two scenarios, the vaccine’s effectiveness remains high decades later, but its effectiveness is estimated to decline very slowly over time.
In the third scenario, immunity begins to decline when measles is no longer endemic, reducing vaccine effectiveness by about 0.04% per year.
For someone born in 1995 who received two doses of the MMR vaccine before the age of 5 and achieved full vaccine protection, the average vaccine effectiveness declines by about 0.04% each year. means maintained.
99.6% at 15 years old 99.2% at 25 years old 98.8% at 35 years old 98.4% at 45 years old
Vaccinated people remain highly protected from measles. In the rare case that a vaccinated person becomes infected with measles, previous research has shown that although they are likely to have milder symptoms than unvaccinated people, they may still be able to transmit the virus. It is suggested that there is. The incidence of measles in vaccinated people was 83% of that in unvaccinated people.
“Our study looks at just a small part of the overall picture of measles cases in the UK. The much bigger problem in terms of measles prevalence is that uptake of the MMR vaccine has fallen in the UK since 2015. “It’s about being there,” says Dr. Ann Saffell. London School of Hygiene and Tropical Medicine.
“Understanding the immunocompromising effects of vaccines can help predict the potential impact of measles in countries where vaccine uptake is declining despite decades of low incidence. It is the best way to limit the impact of measles and protect everyone from this terrible event.”Keeping vaccine uptake as high as possible is important to prevent disease. ”
The authors note that the scenario represents a simplified version of reality and therefore cannot take into account everything that could affect the spread of the disease, and that the accuracy of the model depends on the quality of vaccination data. We acknowledge some limitations of the study, including dependence. Based on.
Furthermore, because measles outbreaks in countries near eradication status are driven by cases in areas or communities with low vaccination coverage, this model is unable to identify these pockets of susceptibility, and therefore future predictions It is considered inappropriate to use this model. Risk of epidemic.
Finally, the authors conclude that testing patterns may have changed between 2011 and 2019, resulting in better identification when vaccinated people become infected and increasing rates of double vaccination. I am aware that it is a possibility. Without access to test data, the authors could not test this assumption.
Dr Adam Kucharski, from the London School of Hygiene and Tropical Medicine, said: ‘Our findings show that the dynamics of measles observed in the UK are consistent with a slow decline in immunity in people who have been vaccinated twice. It shows that there is.”
“Other factors, such as changes in testing patterns over time, may partially explain the increase in the proportion of vaccinated people. However, case reports from other countries and among previously vaccinated people Combined with the consistency and age distribution of the increase in the UK, laboratory studies showing a decline in measles antibodies suggest that a biological explanation is involved.
Further information: Long-term decline in vaccine-induced immunity to measles in the UK: a mathematical modeling study, Lancet Public Health (2024). DOI: 10.1016/S2468-2667(24)00181-6, www.thelancet.com/journals/lan … (24)00181-6/fulltext
Citation: MMR vaccine maintains best protection against measles, but study suggests level of protection decreases slightly over time (September 26, 2024) https://medicalxpress Retrieved September 26, 2024 from .com/news/2024-09-mmr-vaccine-measles-slightly decreasing.html
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