15 - Clinical Update: Influenza 'Flu'


The ‘flu'

Influenza is a virus that is found in animals and humans. Many strains are recognised; an important characterisation of the organism is its propensity to undergo minor or major changes in genetic profile (antigenic drift or shift). Influenza is highly infectious and spreads rapidly in closed communities. The incubation period 1-5 days (average 2-3 days) though may be longer especially in people with immune deficiency.

Most cases in the UK occur during an 8 to 10 week period during the winter. Influenza infection in the general population shows strong seasonal variation with highest incidence in December to March (in the northern hemisphere). WHO figures suggest that, worldwide, 7500000 people die of influenza each year in non-pandemic years. In England, influenza activity is measured according to new GP consultations for influenza and influenza-like illness (ILI), and normal winter seasonal levels are 30–200 cases per 100000 population per week. A small proportion of cases of seasonal influenza are acquired occupationally.
Treatment is with anti-viral agents (Oseltamivir, Zanamivir, or Amantadine).
Prognosis varies according to the strain and the level of herd immunity.

Causal exposures/industries

 Most occupations do not have a greater risk than the general population.
 HCWs who look after infected patients or handle influenza organisms in the laboratory and teachers and care workers in institutions are at increased risk.
 It is important to remember individual susceptibility - the working age population is at increased risk if they have chronic disease (e.g. diabetes, renal failure, cancer, chronic respiratory illness).

Influenza immunisation

The Chief Medical Officer has recommended annual immunisation against seasonal influenza for fit HCWs (i.e. in the absence of specific medical indications). As well as protecting HCWs from occupational transmission, there is reasonable evidence that immunization reduces mortality in their elderly patients
Many OH providers offer influenza immunisation to staff outside the health care sector, even in the absence of increased occupational risk. This is usually justified on the basis that it might reduce sickness absence, although the evidence base for this assumption is incomplete.
Owing to genetic changes, the flu virus constantly changing:
 Antigenic drift: minor changes (natural mutations) in the genes of flu viruses that occur gradually over time.
 Antigenic shift: when two or more different strains combine. This abrupt major change results in a new sub-type. Immunity from previous flu infections/vaccinations may not protect against the new subtype, potentially leading to a widespread epidemic or pandemic.
Because of the changing nature of flu viruses, WHO monitors their epidemiology throughout the world.
Each year WHO makes recommendations about the strains of influenza A and B which are predicted to be circulating in the forthcoming winter. These strains are then included in the flu vaccine developed each year.
Efficacy of the flu vaccine varies from one season to the next. Overall efficacy has been calculated at between 50-60% for adults aged 18 to 65.

There is a lower efficacy in the elderly population although immunisation shown to reduce incidence of severe disease including bronchopneumonia, hospital admissions and mortality.
Provisional end-of-season adjusted vaccine effectiveness estimates for 2018/19 showed an overall effectiveness of 44.3% against a laboratory confirmed infection resulting in a primary care consultation.

Flu vaccine eligibility: 2019/20 flu season:

 All those aged two to ten (but not eleven years or older) on 31 August 2019.
 People aged six months to under 65 years in clinical risk groups.
 All pregnant women (including those who become pregnant during flu season).
 People aged 65 years and over (including those becoming 65 years by 31 March 2020).
 People living in long-stay residential care homes or other long-stay care facilities.
 Carers.
 Household contacts of immunocompromised individuals.
 Frontline health and social care workers with direct patient/service user contact should be provided with flu vaccination by their employer.

Vaccination of clinical risk groups

Increasing flu vaccine uptake in clinical risk groups is important because of increased risk of death and serious illness if people in these groups catch flu.
Vaccine uptake for all clinical risk groups needs to improve as current uptake is some way from the ultimate minimum 75% uptake rate ambition set by WHO.

Flu vaccine uptake by individual clinical risk group in 2018/19 (% ) GP registered patients aged 6 months to under 65 years

When to vaccinate?

Those eligible should be given flu vaccination as soon as vaccine is available so that people are protected when flu begins to circulate in the community, ideally most vaccination should be completed before the end of December before flu circulation usually peaks.
Protection afforded by the vaccine is thought to last at least one influenza season and annual revaccination is important.

Types of flu vaccines

Two main types of vaccine available:
 inactivated –given by injection
 live attenuated –given by nasal application
None of the flu vaccines can cause clinical influenza
 Trivalent vaccines contain two sub-types of Influenza A and one type B virus
 Quadrivalent vaccines contain two sub-types of Influenza A and both B virus types
Live attenuated influenza vaccine (LAIV) - a live attenuated intranasal spray is the recommended vaccine for the childhood flu programme.

The recommended vaccines based on age group

References/ Useful Links