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2 BACKGROUND TO PANDEMIC INFLUENZA
2.1 Influenza viruses and their pandemic potential
Influenza is an acute highly infectious viral infection. There are 3 broad types of influenza virus - A, B and C. Influenza A viruses generally cause more serious illness than B and C and are the cause of most winter epidemics and all known pandemics.
Influenza A viruses can affect a wide range of animal species as well as humans and have a remarkable ability to adapt and change.
The human influenza A viruses which circulate each year undergo frequent genetic modification which result in changes to their main surface antigens (the haemagglutinin (H) and neuraminidase (N)). These year on year changes in influenza viruses are usually minor and are referred to as 'antigenic drift'. This phenomenon explains why influenza vaccines need to be reformulated every year.
From time to time a major stepwise adaptation of the virus or exchange of genetic material between influenza viruses (eg of human and avian origin) results in a major change in the surface antigens called 'antigenic shift'.
Antigenic shift is specific to influenza A viruses and a pandemic of influenza result when a new or re-emerging influenza A virus subtype emerges which is:
- markedly different from recently circulating strains
- able to infect people
- readily transmissible from person-to-person
- capable of causing illness in a high proportion of those infected and
- able to spread widely because few - if any - people have natural or acquired immunity to it.
Whilst such a virus could first emerge anywhere in the world - including the UK - South East Asia is widely considered to be the most likely potential source. Rapid spread is likely to cause an epidemic within the country of origin and becoming a pandemic when it spreads between countries.
The conditions that allow a new virus to develop emerge and spread continue to exist and some features of modern society, such as air travel, could accelerate the rate of spread. Experts therefore agree that there is a high probability of another pandemic occurring, although timing and impact are impossible to predict.
In particular the widespread occurrence of the highly pathogenic avian (bird) influenza virus A/H5N1 in poultry in South East Asia since 2003 is of concern. The A/H5N1 virus, which is extremely contagious and rapidly fatal in most birds, has spread widely in domestic poultry flocks and wild water fowl in the Far East and more recently to other parts of the world, including Europe. The virus has also infected humans (through transmission from bird to human) but to date there has only been limited evidence of person-to-person transmission and even if that has occurred it has been with difficulty and unsustained.
A growing reservoir of infection in birds combined with transmission to more people over time, increases the opportunity for the A/H5N1 virus either to adapt to give it greater affinity to humans or exchange genes with a human influenza virus to produce a completely new virus capable of spreading easily between people and causing a pandemic. The close proximity of humans to poultry and pigs in many parts of the world, eg the Far East, facilitates mingling of human and animal viruses and increases the risk that they may exchange genetic material resulting in a new "reassorted" human strain.
2.2 How influenza spreads
Influenza is one of the most difficult infectious diseases to control because the virus spreads rapidly and easily from person to person. There are two main methods of spread:
- the respiratory route when an infected person talks, coughs or sneezes
- hand-face contact if hands are contaminated with the virus after touching an infected person or surface contaminated with infected secretion.
The disease has a short incubation period in the range of one to four days (typically two to three) and without intervention - or significant immunity in the population - historical evidence suggests one person infects about 1.4 to 1.8 others on average and that influenza spreads particularly rapidly in closed communities such as schools or residential homes. Sufferers are most infectious early in the illness. They may be infectious before the onset of clinical symptoms and are highly infectious for four to five days thereafter. Children and those with conditions that affect their immune systems can be infectious for seven or more days. Some people can be infected without showing symptoms and as they may shed virus, be able to pass on the infection.
Experimental studies suggest influenza viruses can survive in the environment for periods of up to 48 hours and transfer from contaminated surfaces to hands. Experimental data suggest that the virus can survive on hard non-porous surfaces for up to 72 hours and be recovered from soft porous items for up to 24 hours. Studies have also shown that careful hand washing, commercially available alcohol hand disinfectant and domestic cleaning products can easily deactivate the virus.
2.3 What an influenza pandemic might look like
Influenza pandemics have swept the world from time to time throughout history with devastating effect, far in excess of that resulting from 'seasonal flu' which occurs most winters.
Past pandemics have varied in scale, severity and consequence but in general, their impact has been much greater than that of even the most severe winter 'epidemic'. There have also been material differences in the age groups most affected, the time of year they occur and the speed of spread, all of which influence their overall impact. Although little information is available on earlier pandemics, the three that occurred in the twentieth century are well documented. The worst (often referred to as 'Spanish Flu') occurred in 1918/19. It caused serious illness, an estimated 20-40 million deaths worldwide with peak mortality rates in people aged 20-45 and major disruption. Some residual health problems attributed to it lasted for many years thereafter. Whilst the pandemics in 1957 and 1968 were much less severe, they also caused significant illness levels - mainly in the young and the old - and an estimated one to four million deaths.
It is impossible to forecast the precise characteristics, spread and impact of a new influenza virus strain. Modelling suggests that from the time it begins in the country of origin it may take as little as two to four weeks to build from a few to around a thousand cases and could reach the UK within two to four weeks thereafter. Once in the UK, it is likely to spread to all major population centres within one to two weeks, with its peak some 50 days from initial entry.
An influenza pandemic can occur either in one or in a series of 'waves' weeks to months apart. To inform preparedness planning, a profile based on the three pandemics that occurred in the last century and current models of disease transmission has been constructed. That profile is intended to show the fastest overall national progression of a pandemic from the time it becomes the dominant respiratory disease. More locally, epidemics might be over more quickly (6-8 weeks) with a proportionately higher peak.
Single wave profile showing proportion of new clinical cases, consultations, hospital admissions or deaths, by week.
Vaccination or mass treatment with antiviral medicines (assuming their efficacy is similar to that against seasonal influenza) can be expected to modify this profile.
2.4 Predicting the health and wider impact
It is impossible to predict the exact nature, timing or impact of any future pandemic. For planning purposes, impact assessments are derived from a combination of current virological and clinical knowledge, expert analysis, extrapolations from previous pandemics and mathematical modelling.
Mathematical modelling maps the range of possible risks and the robustness of proposed responses. All impact predictions are estimates - not forecasts - made to manage the risks of a pandemic and the actual shape and impact may turn out to be very different.
When pandemics happen, many millions of people around the world can become ill with influenza and a proportion die from the disease itself or complications such as pneumonia. Depending upon the virulence of the virus, the susceptibility of the population and the effectiveness of countermeasures, up to half the population could have developed illness and between 5,100 and 63,700 additional deaths could have occurred in Scotland by the end of the pandemic.
In the absence of early or effective interventions, society is also likely to face much wider social and economic disruption, threats to the continuity of essential services, lower production levels, shortages and distribution difficulties. Individual organisations may also suffer the pandemic's impact on business and services. Difficulties in maintaining business and service continuity will be exacerbated if the virus affects those of working age more than other groups and fear of infection, illness, care providing responsibilities, stress, bereavement and potential travel disruption are all likely to lead to higher levels of absence. Staffing is therefore the critical element in business/service continuity plans.
In addition to maintaining business continuity, organisations will need to consider the security of premises, including manufacturing plants. High levels of public and political concern, scrutiny and demands for advice and information are also inevitable at all stages of an influenza pandemic. An effective communications strategy that provides timely advice and information on the situation in the UK and in other countries forms a key part of the management strategy.
Given the lack of relevant information, assessments of impact on the overall UK economy are necessarily simplistic and can only be illustrative. One such illustrative assessment suggests that illness-related absence from work by 25% of employees (only half of what may be expected in a widespread pandemic) could reduce the year's GDP by between £3bn and £7bn. Between 0.3% - 0.6% additional premature deaths could cause a further reduction of £1bn to £7bn depending on whether case fatality rates are low (0.37%) or high (2.5%) and whether earnings or gross output are used in the calculation. Longer term, the impact of premature death could reduce future lifetime earnings by £21bn to £26bn at a low and by £145bn to £172bn at a high case fatality rate.
Further information on the principles underlying the use of modelling in preparing for an influenza pandemic and some of the results thus far are available at www.dh.gov.uk/pandemicflu
Further advice on business continuity aspects is available at www.ukresilience.info/ccact/index.shtm
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