Modelling the importation risk of measles during the Hajj

A major resurgence of measles has occurred in the past 2 years.

Measles is one of the most contagious diseases, with a reproduction number of around 20. Although a very effective vaccine against measles is available, vaccine coverage is low in an increasing number of countries, including many Muslim countries.

The concern is that measles could be introduced via pilgrims to the Hajj and subsequently spread rapidly as a result of the overcrowded situation of the Hajj rituals.

The 20 leading countries from where Hajj pilgrims travel are listed in the appendix (p 2), and together with Saudi Arabia, they account for more than 80% of all pilgrims. More than 90% of all pilgrims arrive by airplane (eg, within 24 h after departure from their home country). The majority of pilgrims are born before 1957, and are, therefore, considered measles-immune; however, a non-negligible number of pilgrims are born after 1957, and might hence be susceptible to measles. Using travel volume data, we aimed to estimate the number of measles cases imported into Saudi Arabia for the Hajj 2019 (appendix p 1).

Briefly, we obtained data on measles vaccine coverage, measles incidence (reported measles cases divided by population size), and number of pilgrims from each of those 20 countries. On the basis of these data, we calculated the expected number of cases exported from these countries to Saudi Arabia. We assumed that 70% of pilgrims were measles-immune (born before 1957) and calculated the number of susceptible pilgrims based on the vaccine coverage (appendix p 1).