Comparative community burden and severity of seasonal and pandemic influenza: results of the Flu Watch cohort study.
BACKGROUND
Assessment of the effect of influenza on populations, including risk of infection, illness if infected, illness severity, and consultation rates, is essential to inform future control and prevention. We aimed to compare the community burden and severity of seasonal and pandemic influenza across different age groups and study years and gain insight into the extent to which traditional surveillance underestimates this burden.


METHODS
Using preseason and postseason serology, weekly illness reporting, and RT-PCR identification of influenza from nasal swabs, we tracked the course of seasonal and pandemic influenza over five successive cohorts (England 2006-11; 5448 person-seasons' follow-up). We compared burden and severity of seasonal and pandemic strains. We weighted analyses to the age and regional structure of England to give nationally representative estimates. We compared symptom profiles over the first week of illness for different strains of PCR-confirmed influenza and non-influenza viruses using ordinal logistic regression with symptom severity grade as the outcome variable.


RESULTS
Based on four-fold titre rises in strain-specific serology, on average influenza infected 18% (95% CI 16-22) of unvaccinated people each winter. Of those infected there were 69 respiratory illnesses per 100 person-influenza-seasons compared with 44 per 100 in those not infected with influenza. The age-adjusted attributable rate of illness if infected was 23 illnesses per 100 person-seasons (13-34), suggesting most influenza infections are asymptomatic. 25% (18-35) of all people with serologically confirmed infections had PCR-confirmed disease. 17% (10-26) of people with PCR-confirmed influenza had medically attended illness. These figures did not differ significantly when comparing pandemic with seasonal influenza. Of PCR-confirmed cases, people infected with the 2009 pandemic strain had markedly less severe symptoms than those infected with seasonal H3N2.


CONCLUSIONS
Seasonal influenza and the 2009 pandemic strain were characterised by similar high rates of mainly asymptomatic infection with most symptomatic cases self-managing without medical consultation. In the community the 2009 pandemic strain caused milder symptoms than seasonal H3N2.


BACKGROUND
Medical Research Council and the Wellcome Trust.
UCL Research Department of Infection and Population Health, University College London, London, UK. Electronic address: a.hayward@ucl.ac.uk.
Andrew C
AC
Hayward
UCL Research Department of Infection and Population Health, University College London, London, UK; Department of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, London, UK.
Ellen B
EB
Fragaszy
Respiratory Virus Unit, Public Health England, Colindale, UK.
Alison
A
Bermingham
Weatherall Institute of Molecular Medicine, University of Oxford, Oxford, UK.
Lili
L
Wang
UCL Research Department of Infection and Population Health, University College London, London, UK.
Andrew
A
Copas
Department of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, London, UK.
W John
WJ
Edmunds
Department of Infectious Disease Epidemiology, Imperial College London, London, UK.
Neil
N
Ferguson
Weatherall Institute of Molecular Medicine, University of Oxford, Oxford, UK; Department of Microbiology and Immunology, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA.
Nilu
N
Goonetilleke
UCL Research Department of Infection and Population Health, University College London, London, UK.
Gabrielle
G
Harvey
UCL Research Department of Infection and Population Health, University College London, London, UK.
Jana
J
Kovar
UCL Research Department of Infection and Population Health, University College London, London, UK; Centre for Population Health, Burnet Institute, Melbourne, VIC, Australia.
Megan S C
MS
Lim
Weatherall Institute of Molecular Medicine, University of Oxford, Oxford, UK.
Andrew
A
McMichael
UCL Research Department of Infection and Population Health, University College London, London, UK; Department of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, London, UK.
Elizabeth R C
ER
Millett
Health Protection and Influenza Research Group, Division of Epidemiology and Public Health, University of Nottingham, Nottingham, UK.
Jonathan S
JS
Nguyen-Van-Tam
UCL Department of Primary Care and Population Health, University College London, London, UK.
Irwin
I
Nazareth
Respiratory Diseases Department, Public Health England, Colindale, UK.
Richard
R
Pebody
UCL Research Department of Infection and Population Health, University College London, London, UK.
Faiza
F
Tabassum
Respiratory Diseases Department, Public Health England, Colindale, UK.
John M
JM
Watson
UCL Research Department of Infection and Population Health, University College London, London, UK.
Fatima B
FB
Wurie
UCL Research Department of Infection and Population Health, University College London, London, UK.
Anne M
AM
Johnson
Respiratory Virus Unit, Public Health England, Colindale, UK.
Maria
M
Zambon
D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D004279:DNA, Viral / Q000032:analysis*; D004739:England / Q000453:epidemiology; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053122:Influenza A Virus, H3N2 Subtype / Q000235:genetics*; D007251:Influenza, Human / Q000175:diagnosis / Q000453:epidemiology* / Q000821:virology; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D060888:Real-Time Polymerase Chain Reaction; D012042:Registries; D012189:Retrospective Studies; D012621:Seasons; D012720:Severity of Illness Index; D055815:Young Adult
D004279:DNA, Viral