Prevalence of influenza at Hajj: is it correlated with vaccine uptake?

Travellers attending mass gathering events like Hajj are at high risk of acquisition of respiratory viral infection, especially influenza. Following the official recommendation from the Saudi Ministry of Health in 2005, the coverage rate of influenza vaccine rose to 100% by the year 2009, and thereafter remained above 80% in pilgrims from resource-rich countries. In a meta-analysis of observational studies, influenza vaccine was found to be significantly effective against laboratory-confirmed influenza; therefore, it is interesting to determine whether there has been any change in the prevalence of influenza. Over the last few decades a score of studies have been undertaken to establish the prevalence of influenza among Hajj pilgrims. The studies varied greatly in their designs; most notably, two separate endpoints were used: 'point prevalence' and 'attack rate'. The attack rate of influenza among Hajj pilgrims has remained stable between 4% and 15%, as did its point prevalence between 1.3% and 10%. However, the prevalence of influenza-like illness (ILI) varied widely between 8% and 63%, depending on how ILI was defined. It is therefore sensible to assume that the prevalence of influenza at Hajj has not declined considerably, and vaccine and other preventive measures may have contributed to keep the epidemiology stable. More studies are needed to understand the epidemiology of influenza at Hajj, particularly by assessing the efficacy of influenza vaccine against laboratory-proven influenza in pilgrims.