Hajj pilgrimage is one of the largest annual mass gathering events in the world. Respiratory tract infections, particularly due to viruses such as influenza, are considered as the commonest medical presentations at Hajj. To prevent the occurrence and transmission of respiratory viral infections, including influenza, during Hajj, several preventive measures such as influenza vaccination have been recommended for Hajj pilgrims. However, the available data on influenza vaccine effectiveness (VE) among Hajj pilgrims are derived from sporadic and small studies and the outcome is inconclusive. Therefore, this PhD thesis aimed to assess influenza vaccination rate among Hajj pilgrims and its role in preventing respiratory infections including influenza during Hajj.
At the beginning, we evaluated the uptake of influenza vaccine among domestic pilgrims and explored pilgrims’ attitude towards influenza vaccination in recent Hajj seasons. With the observed increase in the vaccine uptake, it was unclear whether such an increase is associated with a decline in the burden of influenza at Hajj or not. Therefore, a retrospective analysis of raw and published data has been performed to examine the association between the vaccine uptake and the prevalence of influenza-like illness and laboratory-proven influenza during Hajj. After that, a retrospective analysis of data from individual Hajj studies across several years was undertaken by using ‘test-negative’ design to estimate the VE of trivalent influenza vaccine at Hajj. To explore possible explanations for the finding of VE in Hajj pilgrims, data on antigenic characteristics of the viruses were collated for both hemispheres and compared to the vaccine strains. Moreover, we collaborated with molecular biologists to perform phylogenetic analysis and characterization of influenza viruses at Hajj.