In recent years, we have witnessed an upsurge of measles cases – even in wealthy nations with a well-established two-dose routine vaccination program and where coverage levels have been historically high. This complex phenomenon may have diverse causes depending on the region being studied. In high-income countries it is mainly ascribable to vaccine hesitancy, and to clustering of unvaccinated individuals in specific groups of the population.
According to WHO, more than 80,000 people contracted measles in 2018 in the European Region, more than tripling the number in 2017 (over 25,000) which was already high. In response to this alarming trend, Italy and France approved new regulations introducing compulsory vaccination against measles for children before they enter pre-primary and primary school. Parents who refuse vaccination may face fines and exclusion of their children from preschool in Italy, whilst non-vaccinated children will not be admitted to nurseries or schools in France. However it is still unclear if these strategies are enough to eliminate measles in these countries.
The main focus of our work was to evaluate the effect of possible adjustments to existing immunisation strategies, and to estimate the proportion of people who may remain susceptible to measles in high-income countries over time.
We studied seven countries (UK, Ireland, Italy, US, Australia, Singapore and South Korea), characterized by different demographic conditions and vaccination histories. We adopted a mathematical modelling approach to project the fraction of susceptible individuals in these countries for the next 30 years under two different vaccination scenarios:
- Assuming different coverage levels for the two-dose routine programs
- Assuming compulsory vaccination for children before they enter primary school in all countries under study, on top of routine vaccination
Measles elimination is defined as the absence of endemic measles virus transmission in a region or other defined geographic area for ≥12 months. As suggested in the epidemiological literature, in order to achieve and maintain elimination, the fraction of individuals who are susceptible to measles infection should not exceed 6-8% of the population; we assumed this number to be 7.5%.
Current coverage not enough
We found that routine programs with current coverage levels in the next decades are not enough to maintain the herd immunity level required to interrupt measles transmission in the countries studied, with the exception of Singapore and South Korea, where high immunisation rates in children has been achieved in the last 10 years.
All other countries would surely benefit from the introduction of vaccination at school entry on top of current routine immunisation programs, should coverage levels of both routine doses remain around 90%. In particular, from an epidemiological point of view, our analysis clearly shows that targeting 40% of unvaccinated children before school entry would be enough for countries like the UK, Ireland, Australia and the US to reach stable herd immunity levels in the future.
We found that routine programs with current coverage levels in the next decades are not enough to maintain the herd immunity level required to interrupt measles transmission
Clearly, the feasibility, sustainability and social acceptability of a mandatory immunisation program per se have to be carefully evaluated by local governments.
In principle, persistent measles elimination can also be achieved in most of the countries studied by reaching and maintaining coverage levels beyond 95% in both first and second dose programs. However, such a large vaccine uptake has never been observed in most high-income countries and may represent a more difficult task than strengthening immunisation at school entry.
Preliminary data from Italy suggest that vaccination coverage has increased since the introduction of compulsory vaccination at school entry. However, our analysis shows that in Italy neither the enhancement of routine coverage levels nor the vaccination at school entry at 100% coverage would be sufficient to decrease the fraction of susceptible individuals below 7.5%, as a consequence of immunity gaps among adolescents and young adults. Therefore, the achievement of measles elimination in settings similar to the Italian one would also require further efforts to reduce susceptibility in older age groups, caused by past sub-optimal vaccine uptake.
An adjustment of current policies is required in most of the countries studied to reach and maintain a sufficiently high proportion of individuals immune to measles – a key requirement for avoiding future outbreaks.