Influenza and the Hajj: defining influenza-like illness clinically

Recent studies have challenged the Centers for Disease Control and Prevention (CDC) definition of influenza-like illness (ILI) as ‘temperature of ≥37.8 °C and either cough or sore throat in the absence of a known cause other than influenza’.1Babcock H.M. Merz L.R. Fraser V.J. Is influenza an influenza-like illness? Clinical presentation of influenza in hospitalized patients.Infect Control Hosp Epidemiol. 2006; 27: 266-270Crossref PubMed Scopus (98) Google Scholar, 2Thursky K. Cordova S.P. Smith D. Kelly H. Working towards a simple case definition for influenza surveillance.J Clin Virol. 2003; 27: 170-179Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar, 3Rashid H. Rafiq S.M. Defining influenza-like illness.Infect Control Hosp Epidemiol. 2006; 27: 1285-1286Crossref PubMed Scopus (5) Google Scholar We assessed the sensitivity and specificity of this definition in identifying ILI at the Hajj, the yearly pilgrimage of Muslims to Mecca, Saudi Arabia, where an estimated 24 000 pilgrims suffer from influenza each year.4El Bashir H. Haworth E. Zambon M. Shafi S. Zuckerman J. Booy R. Influenza among UK pilgrims to Hajj, 2003.Emerg Infect Dis. 2004; 10: 882-883Crossref Scopus (47) Google Scholar, 5Balkhy H.H. Memish Z.A. Bafaqeer S. Almuneef M.A. Influenza a common viral infection among Hajj pilgrims: time for routine surveillance and vaccination.J Travel Med. 2004; 11: 82-86Crossref PubMed Scopus (132) Google Scholar, 6Gatrad A.R. Shafi S. Memish Z.A. Sheikh A. Hajj and the risk of influenza.BMJ. 2006; 333: 1182-1183Crossref PubMed Scopus (27) Google Scholar During the Hajj 2005 and 2006, we recruited a total of 555 pilgrims with upper respiratory symptoms who presented at our clinics in Mecca during the Hajj or attended local mosques in the UK after the Hajj. After obtaining demographics, symptoms and risk factors, we collected nasal swabs for PCR analysis. Individuals with PCR-confirmed influenza were selected as cases and the others as controls; two controls per case were chosen by random numbers.7Random.org—True Random Number Service. Available at: http://www.random.org (accessed January 2007).Google Scholar Fifty-nine (11%) pilgrims had PCR-confirmed influenza; with the exclusion of 17 incomplete entries, 42 of these were ultimately selected as cases. Eighty-four controls were randomly chosen (Table 1). Twelve (28%) cases met the CDC criteria of ILI, however seven (8%) of the controls also met these criteria (p < 0.01) giving a sensitivity and specificity of the CDC definition in identifying influenza of 28% (12/42) and 92% (77/84), respectively. Its positive predictive value (PPV) was 63% (12/19) and negative predictive value (NPV) was 72% (77/107). The combination of cough, sore throat and subjective fever applied to 28 (67%) cases and 30 (36%) controls (p < 0.01), giving a sensitivity and specificity of this triad of 67% (28/42) and 64% (54/84), respectively. Its PPV was 48% (28/58) and NPV was 79% (54/68).Table 1Characteristics of cases and controlsCasesControlsp ValueSample size4284Median age3943Male:female37:536:2At-risk individuals9 (21%)20 (24%)Vaccinated pilgrims9 (21%)19 (23%)Major symptoms Sore throat37 (88%)67 (80%)0.25 Cough37 (88%)59 (70%)0.03 Subjective fever32 (76%)38 (45%)<0.01 Rhinorrhea28 (67%)61 (73%)0.49 Myalgia7 (17%)12 (14%)0.73 Diarrhea4 (10%)7 (8%)0.82 Headache0 (0%)2 (2%)0.31 Temperature ≥37.8 °C13 (31%)9 (11%)<0.01 Open table in a new tab As in this study, other studies have also demonstrated the low sensitivity of the CDC criteria1Babcock H.M. Merz L.R. Fraser V.J. Is influenza an influenza-like illness? Clinical presentation of influenza in hospitalized patients.Infect Control Hosp Epidemiol. 2006; 27: 266-270Crossref PubMed Scopus (98) Google Scholar and have suggested the triad of cough, subjective fever and fatigue to define ILI;1Babcock H.M. Merz L.R. Fraser V.J. Is influenza an influenza-like illness? Clinical presentation of influenza in hospitalized patients.Infect Control Hosp Epidemiol. 2006; 27: 266-270Crossref PubMed Scopus (98) Google Scholar, 2Thursky K. Cordova S.P. Smith D. Kelly H. Working towards a simple case definition for influenza surveillance.J Clin Virol. 2003; 27: 170-179Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar, 3Rashid H. Rafiq S.M. Defining influenza-like illness.Infect Control Hosp Epidemiol. 2006; 27: 1285-1286Crossref PubMed Scopus (5) Google Scholar however fatigue was not reported by any of our recruits, possibly because pilgrims expect to feel fatigue as a result of strenuous Hajj rituals or as a travel-associated symptom.8Pollard A.J. Murdoch D.R. Fast facts: travel medicine. Health Press, Oxford2001Google Scholar A Canadian study showed that the probability of a positive result of influenza increased with higher temperature,9Boivin G. Hardy I. Tellier G. Maziade J. Predicting influenza infections during epidemics with use of a clinical case definition.Clin Infect Dis. 2000; 31: 1166-1169Crossref PubMed Scopus (278) Google Scholar however recording temperature is not always practical at large gatherings. Furthermore, its measurement may be influenced by the route, thermometer type, phase of infection, and antipyretic use.1Babcock H.M. Merz L.R. Fraser V.J. Is influenza an influenza-like illness? Clinical presentation of influenza in hospitalized patients.Infect Control Hosp Epidemiol. 2006; 27: 266-270Crossref PubMed Scopus (98) Google Scholar, 3Rashid H. Rafiq S.M. Defining influenza-like illness.Infect Control Hosp Epidemiol. 2006; 27: 1285-1286Crossref PubMed Scopus (5) Google Scholar We suggest, therefore that the triad of ‘cough, sore throat and subjective fever’ is used to define ILI at the Hajj or other mass gatherings (e.g., Olympic games, FIFA World Cup), which is simple, specific and more sensitive than the CDC definition. This work was supported by a Department of Health grant. We thank the staff at the Respiratory Virus Unit and the London South Specialist Virology Center, Health Protection Agency, London, for doing the PCR tests. Conflict of interest: SS chaired the Health and Medical Committee of the Muslim Council of Britain (until August 2006). RB has received support from Roche, CSL and Sanofi to attend scientific meetings.

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