Current vaccination policies may not be enough to prevent measles resurgence

To successfully achieve and maintain measles elimination in these countries in the medium to long term, further country-specific immunisation efforts may be needed in addition to current strategies. Measles elimination has been defined as the absence of endemic measles transmission in a region or other defined geographic area for twelve months or longer.

A team of researchers at the Bruno Kessler Foundation and Bocconi University, Italy used a computer model to simulate the evolution of measles immunity between 2018 and 2050 in seven countries; Australia, Ireland, Italy, Singapore, South Korea, the UK and the US. The authors focused their analysis on countries with a routine two-dose measles vaccination programme and a high primary school involvement rate, but with different demographics and vaccination histories. The aim 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.

The authors' projections up until 2050 suggest that if current vaccination policies remain unchanged, the proportion of the population susceptible to measles would only remain below 7.5% in Singapore and South Korea, two countries which had high vaccination coverage in the past. Previous research estimated that the proportion of the population that does not have immunity (maximum susceptibility) needs to be 7.5% or less for measles to be eliminated.

In 2018, the proportion of the population susceptible to measles infection in the countries under study ranged from 3.7% in the UK to 9.3% in Italy (the only country where the proportion was found to be higher than 7.5%). In Australia, Ireland, the UK and the US, vaccination from routine programmes would need to continuously cover more than 95% of the population to keep the proportion of susceptible individuals below 7.5% until 2050.

Dr. Filippo Trentini, the first author said: "In recent years, we've witnessed a resurgence of measles cases even in countries where, according to World Health Organisation guidelines, elimination should already have been achieved. This resurgence is due to suboptimal vaccination coverage levels. In Italy, where measles incidents rates were among the highest, the government has made measles vaccination compulsory for children before they enter primary school. We investigated the potential of this and other policies to reinforce immunisation rates in seven high-income countries."

Co-author Dr. Stefano Merler added: "Our results suggest that most of the countries we have studied would strongly benefit from the introduction of compulsory vaccination at school entry in addition to current immunisation programmes. In particular, we found that this strategy would allow the UK, Ireland and the US to reach stable herd immunity levels in the next decades, which means that a sufficiently high proportion of individuals are immune to the disease to avoid future outbreaks. To be effective, mandatory vaccination at school entry would need to cover more than 40% of the population."

In Italy, the fraction of susceptible individuals by 2050 is projected to be 10%, even if coverage for routine vaccination reaches 100%, and additional vaccination strategies targeting both children at school entry and adults may be needed to achieve elimination.