Measles are on the rise again. Countries across the globe are facing new outbreaks and in Belgium, Sciensano is calling on all adults to check their vaccination status and be mindful of the symptoms. What’s driving this new surge in cases?
Before the first vaccines for measles hit the market in the 1960s, the contagious childhood disease was the leading cause of child mortality, claiming more than 2.5 million lives every year. Thanks to large-scale vaccination campaigns, that number has drastically decreased to “only” 142,000 deaths in 2018.
However, in the past five years, we have seen a worldwide increase in the number of cases, with outbreaks in various European countries, the USA, and even on small islands in the Pacific Ocean like Samoa.
In Belgium, too, there have been more and more measles cases reported, with a steady rise each year since 2016. 2020 saw a sudden decline due to the coronavirus pandemic and the extensive measures to halt viral spread. With the pandemic now mostly behind us, the possibility of facing major local measles outbreaks is back on the table.
Mathematics in action
Ten years ago, we developed a model to map the risk of measles outbreaks. We looked at the presence of antibodies in the blood, at vaccination coverage, and at contact patterns. Other researchers have applied the same approach to look at the situation in different countries. All this research suggests that if we want to eliminate measles, we may need much higher levels of immunity than we previously thought.
Whereas in the past, most adults used to have natural immunity against measles through exposure to the virus itself, the relative proportion of people immune through vaccination instead of infection is growing rapidly. Fewer and fewer people have experienced high circulation of the measles virus and more and more people are only protected against measles through vaccination.
However, the immunity built up through vaccination is not entirely identical to that built uo through exposure. Unfortunately, the data we routinely collect from blood analyses fall short of providing a clearer picture of the differences between the two. This makes it difficult to accurately estimate the real risk of outbreaks.
Some analyses suggest that the immunity built up after vaccination will wane for some, over a long period of twenty years, making some people susceptible to infection again. At the same time, exposure to the virus can also act as a natural booster for those who have already been vaccinated.
What makes the story even more complex is that the vaccination strategy has changed over the years. In Belgium, before 1985, a limited number of children received a one-time measles vaccine. From 1985, the measles vaccine (in combination with a mumps and rubella vaccine, the so-called MMR vaccine) was routinely adminstered to all children when they turned 1 year old. Ten years later, in 1995, a second dose was added for young teenagers. More recently, we see a growing group of people who are critical of vaccination and therefore do not vaccinate their children.
Then there’s also COVID-19… The pandemic has had enormous consequences for routine vaccination schemes worldwide. The true impact of that setback will only be felt over time, but we can don’t have to wait to take action and start mitigating those effects.
Evidence-based policy
We have come a long way, and the measles vaccines have already saved tens of millions of lives worldwide. However, the goal of completely eradicating the disease now seems increasingly out of reach.
This situation calls for targeted action. Firstly, we need to restore the vaccination rate, including catch-up campaigns for those who never received a double dose or through additional doses for those whose vaccination status is unclear.
In addition, we need a clearer picture of who is susceptible. Analyzing blood samples is a crucial step. Unfortunately, we only have limited data on immunity built up in blood samples across the past decades. A broader rollout of such serological analyses could provide a wealth of information about the long-term effectiveness of current vaccines and the differences between certain vaccination strategies.
Especially in countries where the vaccination rate is already high, like in Belgium, it is precisely this fine-graind information that will help us close the last significant immunity gaps and make sure we eliminate measles once and for all.
What the data say
Who’s at risk?
This graph shows how often new measles cases occur in a specific age group and city after the disease is introduced, considering the city’s unique layout and population distribution. There are two major groups susceptible to measles in our country. Firstly, infants younger than one year old, who have not yet received their first vaccine dose. The other group consists of those in their thirties and forties. In this age category, not everyone received two doses of the measles vaccine as a child because the vaccination campaign had only just begun. Due to the rollout of the vaccination, the spread of measles gradually decreased. As a result, those in their thirties and forties have not built up natural immunity, unlike their parents who did contract the disease.
Further reading
- https://www.eurosurveillance.org/content/10.2807/1560-7917.ES2015.20.1.20998
- https://www.sciencedirect.com/science/article/pii/S0264410X22005795
Source: https://www.simid.be/news/whats-behind-the-surge-in-measles-cases/