Pandemic H1N1 influenza at the 2009 Hajj: understanding the unexpectedly low H1N1 burden

Abstract

The Kingdom of Saudi Arabia (KSA) administered the 2009 Hajj, the largest annual mass gathering of people from about 160 countries, amid a declared global H1N1 pandemic. The unexpectedly low incidence of H1N1 during the Hajj is of concern.1 The official report for the 2009 Hajj refers to laboratory-confirmed, symptomatic pilgrims who sought clinical care in Hajj-related clinical facilities during the five-day period of Hajj. Because of the short duration of the Hajj rituals (5 days), the commitment of pilgrims to perform the Hajj rituals, and the mild clinical course of 2009 H1N1 pandemic influenza for most infected persons, it is possible that symptomatic pilgrims may have deferred contact with the healthcare system, unless symptoms became severe. This could explain the high case fatality observed during the Hajj despite the low disease burden. Furthermore, illnesses that developed after the strictly defined five-day Hajj rituals are not included in the statistics released on the final day of Hajj. For the 2009 Hajj, KSA collaborated with the US Centers for Disease Control and Prevention, the World Health Organization Eastern Mediterranean Regional Office, and the US Naval Medical Research Unit #3 based in Cairo on H1N1 surveillance. The activities included clinic-based sentinel laboratory surveillance in Hajj-specific clinics and community-based laboratory surveillance activities at the Jeddah Airport Hajj Terminal during the arrival and departure of pilgrims. Given the one- to four-day incubation period of influenza viruses, the departure survey of about 3000 pilgrims (about 0.01% of an estimated 2.5–3 million pilgrims) conducted during the first five days after the end of Hajj will help assess the burden of influenza infections acquired during the preceding Hajj period. Because the laboratory tests in these surveillance activities covered tests for all influenza viruses, real-time reporting of the test results was not feasible. Participants in any mass gathering may not all travel in the immediate period before or after the mass gathering event, or may use multiple modes of transportation that limits surveillance data capture. Extended surveillance timeframes implemented at diverse entry and exit points are required to capture indicators of disease spread or amplification. Post-event surveillance should be an integral component of mass gathering surveillance systems to understand the full extent of diseases acquired during an event. Given that 75% of the pilgrims originate from the 40 lower or low-to-middle income countries2 with limited surveillance infrastructure, post-Hajj surveillance in home countries of pilgrims remains a challenge. Rapid one-time disease surveillance among diverse international populations is feasible during mass gatherings and can greatly enhance and ultimately assure global public health security.3