Prompt recognition of sustained community transmission in the earliest phase of an epidemic of a novel influenza virus is important to improve short term predictions, to guide public health decisions on switching from a policy of containment to one of mitigation,1
and for fulfilling criteria for WHO phases of pandemic alert.2
When laboratory confirmed cases of influenza A/H1N1 2009 (swine flu) in England steadily increased during May and June 2009, it was unclear whether community transmission had begun and whether the transmission rate in the country was increasing. A number of factors contributed to this uncertainty.
Foremost was the concern that if initial laboratory testing capacity concentrated on ill people with links to affected countries or who were in close contact with patients whose symptoms had been microbiologically confirmed,3
then surveillance would fail to recognise the extent to which local community transmission was occurring.4
In addition, existing syndromic surveillance systems might not have been sufficiently specific during the early stages of the epidemic. The influenza syndromic surveillance capability of the Health Protection Agency (HPA) includes the Royal College of General Practitioners (RCGP) Weekly Returns Service5
and the QSurveillance system (University of Nottingham and Egton Medical Information Systems Ltd),6
which analyse anonymised morbidity data automatically extracted from networks of participating general practices. Since the beginning of the influenza A/H1N1 2009 epidemic, general practice diagnoses of influenza-like illness and a range of other respiratory tract infections (rates per 100
000 population) have been monitored on a daily (QSurveillance) and weekly basis (QSurveillance, RCGP), providing information for regular situation reports.7
Throughout May and most of June 2009, however, general practice consultation rates for influenza-like illness remained well below the upper limit for “baseline activity” of 30 cases per 100
000 population. Only during the second half of June did a clear increase in consultation rates became apparent in either the RCGP system or the QSurveillance system.8
One response to the perceived poor specificity of syndromic surveillance for recognising illness caused by the novel virus was the re-introduction of virological testing. During each influenza “season,” a programme of integrated nose and throat swabbing is conducted within the RCGP system.9
In addition, the HPA operates a Regional Microbiological Network general practice spotter scheme.10
Both systems were re-instigated to enhance community based virological surveillance. In May 2009, however, there was unease that the geographical scope of these sentinel virological sampling systems may have been too limited so that sustained community transmission may have been underascertained once begun and if concentrated in a few places. Moreover, once members of the public with suspect flu symptoms were advised in early May 2009 not to attend general practitioner surgeries or accident and emergency departments unless they were seriously ill or advised to do so,11
the case mix of those receiving primary care from their general practitioners was likely to have changed.
NHS Direct is a multi-channel health advice and information service for the population of England.12
In the telephone channel, operators use a series of clinical assessment algorithms to evaluate the symptoms of each patient and the predominant symptom is recorded. The service is nurse led, and other health professionals—such as pharmacists—are used where appropriate. Callers are triaged, and their reported symptoms and the severity are evaluated to produce recommended call outcomes that include advice for self care, referral to an emergency department, referral to urgent general practitioner care, or referral to routine general practitioner care. All outcomes are dependent on the seriousness of the call and the “risk status” of the caller—that is, whether they are young or old, or have other illnesses. The service is accessible all day, every day and provides a reliable and continuous feed of data that are utilised by the HPA Real-time Syndromic Surveillance Team to form the basis of the NHS Direct and HPA syndromic surveillance system.12
Previous work has demonstrated that call data are sensitive to increases in community transmission of a range of pathogens, including influenza. Daily numbers of calls recording cold or flu symptoms, or both (aggregated across all ages), and fever (in patients aged 5-14 years) provide early warning of the beginning of community influenza transmission in winter.13
During the swine flu epidemic, the existing NHS Direct and HPA syndromic surveillance system was augmented on 28 May 2009 with a scheme of self sampling and virological testing of telephone callers that had been piloted during the winter of 2003-2004.14
The clinical assessment algorithm used by NHS Direct for callers concerned about swine flu distinguished a subset of patients with generally uncomplicated illness who had no travel associated risk, no contact with other suspected or confirmed influenza A/H1N1 2009 cases, and had a “self care” call outcome.
In addition, laboratory confirmation of influenza A/H1N1 2009 infection before the end of May 2009 depended on the results of tests that were only available at the national reference laboratory. From 1 June 2009, however, suitable testing facilities became available throughout the HPA’s network of regional laboratories. Thereafter, the volume of clinical testing increased rapidly, as did the numbers of laboratory confirmed diagnoses.
The aim of our study was to compare, throughout June 2009 when the swine flu epidemic was taking hold, the weekly information from virological testing of self obtained samples from NHS Direct callers with information from laboratory testing of swabs taken from patients assessed with the clinical algorithm for management of patients with influenza-like illness to see what conclusions might be drawn about the timing of the onset of sustained community transmission. More specifically, we sought to know whether at the beginning of an epidemic the results from the swabs of NHS Direct callers could improve ascertainment of the onset and extent of sustained community transmission of a new influenza virus.