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Influenza surveillance: primary care has a vital role

Brian Smyth
Regional Epidemiologist
Communicable Disease Surveillance Centre (Northern Ireland)
Belfast City Hospital
From 1988-98 Consultant in Communicable Disease Control, Northern Health and Social Services Board, Northern Ireland

At an individual level, influenza is an unpleasant acute illness often characterised by an abrupt onset of fever, headache, myalgia, coryza, sore throat and cough. Although the cough may be severe and prolonged and the acute phase require bed rest, recovery is usual in 2-7 days.(1) Secondary bacterial infection is common, causing bronchitis and ­pneumonia.

However, it is at the population level that influenza derives its public health importance. Influenza can spread rapidly, has epidemic and pandemic potential, and can cause considerable widespread morbidity. During major outbreaks, the elderly and those debilitated by chronic cardiac, respiratory, renal or metabolic disease, anaemia or immunosuppression are at most risk from severe disease and death.

Increased influenza activity therefore places a sudden rise in demand for health and social care on an often- stressed service. In view of this, the UK Health Departments published contingency pandemic plans in 1997.(2) Primary care has key roles in influenza surveillance and protecting those most at risk of influenza- related complications.

Types of influenza
Three types of influenza virus exist: A, B and C. Type A cause most epidemics and can cause worldwide epidemics (pandemics). Influenza A virus undergoes major and minor antigenic changes, so there are always immunologically susceptible individuals in the population.

Pandemics occur when completely new subtypes circulate. The main pandemics last century were in 1918, 1957 and 1968. The largest, in 1918, caused approximately 20 million deaths worldwide, including deaths in young adults, and it is estimated that 23% of the UK population developed influenza that year.(2)
In the UK, influenza A activity increases during most winters, although not necessarily to epidemic levels. Type B can also cause outbreaks, usually between outbreaks of influenza A. However, these tend to be less extensive and are usually associated with less severe illness. Type C is linked with sporadic cases and minor localised outbreaks and is of relatively little importance.

As influenza activity varies each year in severity and time of onset, surveillance arrangements are an integral part of contingency planning. Primary care has an important role in enhanced influenza surveillance as it will be the first part of the health service to note increased respiratory illness in the community. Hospital admissions for influenza-related complications occur later, and it is mainly the hospitalised patients who have laboratory investigation.

Surveillance arrangements need to detect or determine in a timely manner:

  • An increase in community respiratory illness.
  • Whether this is due to influenza virus.
  • Whether the composition of the current influenza vaccine offers protection against circulating strains.
  • The emergence of a new strain of influenza.

Surveillance in the UK and Ireland
In each part of the UK and Ireland the national centres coordinate influenza surveillance schemes, which combine information from primary care and the laboratories. A network of sentinel general practices report the number of consultations for influenza and influenza-like illness per week.

The reporting season is usually from week 40 (early October) to week 20 (mid May). This information is collated and expressed per 100,000 population. Methodologies and case definitions vary slightly, but the overall approach is similar. Trends in NHS Direct calls are also utilised to take into account changes in the provision of primary care in recent years. In Northern Ireland, similar data are forwarded weekly by out-of-hours GP Cooperatives. NHS Direct calls can also be disaggregated into those for "colds/flu".

Some of these schemes, such as the RCGP Birmingham Research Unit Weekly Returns Service,(3) are well established, which has facilitated establishing baseline trends and thresholds to determine when influenza/influenza-like activity has exceeded "normal seasonal" levels and reached "higher than average" or "epidemic" levels.

Some practices also submit nasal and/or pharyngeal swabs from patients presenting with clinical influenza or acute respiratory illness for laboratory analysis. This augments routine diagnostic samples submitted from hospitalised patients.

All of the UK data is collated, linked as appropriate with international data, interpreted and summarised for inclusion in the Public Health Laboratory Service (PHLS) weekly influenza report(4) or equivalent.

Trends are therefore quickly detected and reported to the Departments of Health and health authorities and trusts. Through this mechanism, Health Departments can be advised when influenza is circulating so that zanamivir can be considered for suitable patients.(5)

The 2001/02 winter in the UK was relatively quiet in terms of influenza activity. GP consultations for influenza and influenza-like illness peaked in early/mid February, although rates were within "baseline activity". Consultation rates peaked in Northern Ireland in mid January. Influenza virus strains isolated were antigenically similar to current vaccine strains.(4)
Influenza vaccination
As a result of antigenic changes to the influenza virus, the World Health Organization (WHO) issues advice annually on the formulation of influenza vaccine to be used during the forthcoming winter.

The vaccine composition for the Northern Hemisphere for 2002/03 is considered to offer good protection against the recently identified H1N2 subtype.(6) This new strain is thought to have arisen by reassortment of influenza A strains (H1N1) and (H3N2).

Influenza vaccination offers 70-80% protection when there is a good match between the vaccine and circulating influenza strains. Even if the vaccine does not fully protect against the disease, the severity of illness and frequency of serious complications are reduced.

Until recently, UK influenza vaccine uptake has been low, with one study estimating an uptake rate in the over-65-year age group to be 43.9% in 1996/97.(7) There is now much greater emphasis on the promotion of influenza vaccine following the government's decision to recommend influenza vaccine to all aged over 65 years: national targets have been set, and coordinated public and professional awareness programmes have been established at national and local levels.

During the 2000/01 winter, vaccine uptake in the over-65 population was 65% in England. In Northern Ireland, vaccine uptake among this age group during 2001/2 was 72%, with 61% of practices achieving an uptake rate greater than 70%.

The identification of many "at risk" patients, particularly those aged under 65, is possible only through primary care, for example by using chronic disease registers and prescribing data.

There have been major enhancements to influenza surveillance and the promotion of influenza vaccine over the last five years. The challenge now is to maintain this momentum each year, in the midst of further organisational change, not just for optimum clinical care and professional practice but as an integral part of contingency preparedness.



  1. Chin J, editor. Control of Communicable Diseases Manual. Washington: American Public Health Association; 2000.
  2. Department of Health. UK Health Departments' multiphase contingency plan for pandemic influenza. London: Department of Health; 1997.
  3. RCGP Birmingham Research Unit. Weekly Returns Service. Available from URL: http://www.rcgp-bru.
  4. Public Health Laboratory Service. Disease facts: influenza. Available from URL: influenza/flu.htm
  5. National Institute for Clinical Excellence. Guidance on the use of zanamivir in the treatment of influenza. London: National Institute for Clinical Excellence; November 2000.
  6. WHO. Experts decide content of 2001-2002 "Northern Hemisphere" influenza vaccine. Available from URL: en/pr2001-08.html
  7. Irish C, Alli M, Gilham C, Joseph CA, Watson JM. Influenza vaccine uptake and distribution in England and Wales, July 1989-June 1997. Health Trends 1998;30(2):51-5.