ArticlesContribution of influenza and respiratory syncytial virus to community cases of influenza-like illness: an observational study*
Introduction
Three known types of influenza virus (A, B, and C) currently circulate in the human population, types A and B being associated with clinically important respiratory illness. Influenza A virus is subtyped according to the properties of its surface antigens (haemagglutinin [H] and neuraminidase [N]), and two subtypes—H1N1 and H3N2—have been circulating in human beings for the past 20 years. Seasonal circulation of influenza occurs every winter, and lasts for between 8 and 12 weeks in England and Wales.1
Respiratory syncytial virus (RSV) is best known for its tendency to cause bronchiolitis in infants, but it can infect all age-groups, causing upper and lower respiratory-tract infections ranging in severity from subclinical infections to pneumonia and death.2 RSV is subtyped into RSV A and RSV B according to the antigenic properties of its proteins, and both subtypes circulate in hospitalised infants and elderly individuals in institutions.3 Further classification of RSV has been proposed on the basis of partial sequence analysis,4 although the biological significance of such classification is unclear. In a study of community-acquired pneumonia in adult hospital inpatients, RSV was among the four most frequent laboratory-confirmed causes of infection, along with Streptococcus pneumoniae, influenza virus, and Mycoplasma pneumoniae.5
Acute respiratory infection from any cause affects 30% of elderly people each winter.6 However, elderly patients often present late, are not investigated microbiologically, and do not shed as much virus as children, the result being that accurate laboratory diagnosis is difficult.7 About 5% of elderly people are thought to become infected with RSV every year, and estimates of the rate of pneumonia as a complication of such infections range from 10% to 55%.6 Mortality associated with RSV has been recognised for some time,8 and estimates range from 3–5% in nursing homes to 10–20% in hospitals. Modelling with regression analysis suggests that deaths associated with RSV could be 60–80% higher than those associated with influenza in the community.9
The need to improve understanding of the burden of illness, transmission, and detection of RSV in adults in community settings has become more evident with the advent of interventions for treatment of influenza, with which RSV can be confused clinically, and the development of vaccines for RSV infection.10 Linked clinical-virological surveillance for influenza has been carried out in England and Wales since 1993.11 This approach has consistently proved its value for early warning of influenza epidemics in England and in several European countries.12 We examined virological specimens submitted from patients in the community over the winter periods of 1996–97, 1997–98, and 1998–99 to assess the relation between RSV and influenza and to determine whether strains of RSV circulating in adults are similar to those circulating in children.
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Methods
Clinical data on the rates of influenza and influenza-like illness and acute bronchitis were collected by continuous morbidity recording in about 75 sentinel practices distributed across England and Wales, and covering 700 000 people of all ages. All new episodes of illness were recorded, indexed, and reported weekly to the Royal College of General Pracitioners research unit in Birmingham where they were analysed to provide age-specific incidence data, presented as new consultations per 100 000
Results
Nasopharyngeal swabs were obtained from 2226 cases of influenza-like illness presenting to the subgroup of general practitioners included in the swabbing programme. This number represented about 7·5% of the total consultations for influenza-like illness in the sentinel network during the three winter seasons (6% in 1995–96, 7% in 1996–97, and 11% in 1997–98). Samples were obtained from all age-groups in all three winters. In each winter, the greatest number of swabs was submitted from adults
Discussion
Synchronous influenza detection by reverse transcription PCR, virus isolation, or both, and correlation with consultations for influenza-like illness is in agreement with previous experience,11, 17 and confirms the validity of this approach for monitoring virus circulation in the community. In each of three successive winters, RSV was identified in community-dwelling individuals of all agegroups diagnosed with influenza-like illness. RSV is therefore an important pathogen contributing to the
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The sequence information generated from this study is deposited in EMBL/GenBank, accession numbers AJ410785–AJ410856, and AJ314644