Mass gatherings, such as this year's Beijing Olympic Games, typically involve travel
and extended close contact among participants and spectators. Influenza is one of
the most frequently identified respiratory viruses in travellers,
1
and the high density of people at mass events further increases the risk of transmission.
Despite this, the epidemiology of influenza outbreaks in large congregations is poorly
understood. Variability of venues and the infrequency of mass events may have been
barriers for systematic study of communicable diseases in these situations. However,
because the venue does not change and the event occurs every year, the Islamic Hajj
pilgrimage to Mecca, Saudi Arabia, does provide an excellent opportunity to research
infectious diseases that not only affect a mass event but also have the potential
for pandemic spread.
Infectious diseases at the Hajj have been reported for centuries. During the first
Hajj in 632AD, the pilgrims had febrile illness known locally as “Yethrib fever”,
which is now believed to be malaria.
2
Subsequently, major epidemics such as plague and cholera have been reported. Over
the past decades there have been several intercontinental outbreaks of meningococcal
disease, first caused by serogroup A and later by W135.
3
Currently, respiratory infections are the commonest illnesses encountered at the Hajj
and include, among others, influenza, pertussis, and tuberculosis.
3
Depending on study design, the occurence of seasonal influenza at the Hajj ranges
from 6% to 38%, affecting both domestic and overseas pilgrims.4, 5, 6 So far, no human
case of avian influenza A (H5N1) has been reported at the Hajj or in Saudi Arabia,
but cases of H5N1 in human beings (table
) have been reported in Indonesia, Pakistan, and Turkey, countries from which many
people make the annual pilgrimage to Mecca. Counting Bangladesh, Iraq, Egypt, Azerbaijan,
Nigeria, and Djibouti, a total of nine countries with predominantly Muslim populations
have already been affected by the virus. As of July 25, 2008, over half (214/385)
of the reported number of H5N1 influenza cases have been in Muslims.
7
Furthermore, there was an outbreak of H5N1 influenza in Saudi poultry earlier last
year.
Table
Muslim countries with H5N1 influenza that send pilgrims on Hajj
Population (millions)
Proportion of population who are Muslim
Number of pilgrims per year
Number of cases of H5N1 influenza
7
H5N1 case fatality (%)
7
Indonesia
237·5
86·1%
200 000
135
110 (81%)
Pakistan
167·8
97%
150 000
3
1 (33%)
Turkey
71·9
99·8%
120 000
12
4 (33%)
Nigeria
138·3
50%
90 000
1
1 (100%)
Egypt
81·7
90%
75 000
50
22 (44%)
Bangladesh
153·6
83%
65 000
1
0 (0%)
Iraq
28·2
97%
30 000
3
2 (67%)
Azerbaijan
8·2
93·4%
4000
8
5 (63%)
Djibouti
0·5
94%
2000
1
0 (0%)
Total
..
..
736 000
214
145 (68%)
The recent account of likely human-to-human transmission of H5N1 influenza in China,
reported by Wang and colleagues,
8
alerts us to the possibility of a pandemic arising from mass events such as the Hajj
pilgrimage. In Wang and colleagues' report the index case was a young salesman whose
source of exposure could not be ascertained; for the second case, which concerned
his retired father, the infected son was the only apparent source of exposure. Genomic
sequencing showed that the viruses from both patients were essentially homologous.
This is the third example of potential person-to-person transmission of H5N1 influenza
after the Thai and Indonesian family clusters.
8
What can be done to prevent an influenza outbreak during the Hajj? An awareness campaign
and better surveillance can have an important role. Severe acute respiratory syndrome
(SARS) is a good example of the success of this approach. During the 2003 episode,
of 8000 cases of SARS reported worldwide at least 142 were travel-associated.
9
However, tight measures taken by Saudi authorities, which included applying a ban
on visitors from SARS-hit countries, home quarantine for domestic travellers, installation
of thermal cameras in major airports, and ensuring facilities for rapid detection
of SARS virus, prevented establishment of the virus in the country.
10
Initial results from studies of prototype vaccines against H5N1 influenza were disappointing,
with two or more doses, of high antigenic content, being necessary to induce protective
immunity.
11
Also, from our experience with seasonal influenza, we can assume that the H5N1 vaccine
could be ineffective despite good immunogenicity if the pandemic strain does not match
with the vaccine strain(s). Three studies from Pakistan, Malaysia, and Iran have so
far assessed the effectiveness of seasonal influenza vaccine against influenza-like
illness among Hajj pilgrims. The Pakistani and Malaysian studies reported the vaccine
to be effective; however, the Iranian study, which was done over two consecutive influenza
seasons, reported that the vaccine was effective during the 2003 Hajj but not during
the 2004 Hajj.12, 13, 14 Virological surveillance reports suggest that the rate of
influenza in vaccinated pilgrims might not significantly differ from that in unvaccinated
pilgrims.5, 6
Stockpiling antiviral drugs is another important control strategy but resistance can
be a problem. Therefore, a multi-pronged approach will be required to prevent pandemic
influenza at mass gatherings such as the Hajj. Key elements of this approach include
education and awareness, improved surveillance, rapid diagnosis, and containment with
antiviral treatment of cases and close contacts, non-pharmacological interventions
including surgical masks, and, above all, coordinated global research with viral typing
in countries of origin and sharing of these data.
Because the recent report of probable inter-human transmission of H5N1 occurred in
China,
8
the host of this year's Olympic Games, we believe that pandemic influenza should be
an important consideration in public-health planning for the event. With data from
other mass sporting events, we can postulate that respiratory and gastrointestinal
infections will predominate during this year's Olympic season. Respiratory illnesses
were the leading medical encounters during the 2002 Winter Olympics in Salt Lake City,
USA; a hospital electronic medical record-based public-health surveillance system
deployed during the event suggested a substantial increase in influenza during the
games.
15
For the past few years the Hajj has been a winter event. At this time of year, there
is increased risk of human influenza and also the possibility of dual infection with
human and avian viruses, potentially leading to a genetically altered influenza virus
of high virulence and transmissibility. The latest report of likely inter-human transmission
of H5N1 influenza suggests that the virus is slowly becoming more adapted to human
beings.
8
If the transmission among human beings becomes sustained, mass gatherings could help
spread the virus all over the world at a very rapid pace. Therefore, coordinated global
response and research is a high priority.