Influenza & SARS

Information about avian Influenza (Bird Flu)


Influenza Virus

The influenza virus is classified in a number of different ways. The major group classification is A, B and C. All groups can affect humans. Group A can also affect birds and some animals. Subtypes of influenza differ because of proteins on the viral surface (hemagglutinin [H] and neuraminidase [N] proteins). There are 16 different H subtypes and 9 different N subtypes. Many different combinations of H and N proteins are possible.

What is the relevance of H and N?

Over time, the influenza virus changes slightly. This is described as antigenic drift. When the changes in the molecular shape of the virus are slight, the body is still able to recognize the virus and some immunity from previous infection or immunization is retained. The greater the change in the physical structure of the virus, the less effective is the immunity from previous exposure. A significant change in the virus may occur within the same HN subtype, for instance the subtypes responsible for the pandemics of 1918 (H1N1) and 1968 (H3N2) still circulate today. The progressive change in influenza virus, even within the same sub-type, is the reason that influenza vaccination must be repeated annually.

Influenza vaccination

The influenza vaccine is intended to protect against the most common strains circulating in the human population. Currently, this includes 2 strains of influenza A and one of influenza B.

Winter 2005 Vaccine:

A/New Caledonia/20/99(H1N1)
A/California/7/2004(H3N2)
B/Shanghai/361/2002

Because they are different to the current avian influenza virus (H5N1), it is very unlikely that the current influenza vaccination will give any protection against avian influenza.

Is there any benefit to influenza vaccination in the prevention of avian influenza?

There are two specific reasons for immunizing against influenza A when considering the prevention of avian influenza.

1. If the number of people suffering from influenza can be reduced, there will be less influenza virus circulating in the world. This will reduce the probability of an individual either human or animal being infected by influenza A and avian influenza at the same time, thus reducing the risk of a sudden change to a more easily transmissible form of avian influenza.

2. If the number of people suffering from influenza can be reduced, the overall health and immunity of the population will be increased reducing the likelihood that any new epidemic of infectious disease will spread.

Who should have influenza vaccination?

The Hong Kong Centre for Disease Control recommends vaccination for the following groups:

• Persons aged 65 or above
• Children aged 6 months to 2 years
• Women in the second and third trimester pregnancy
• Individuals with chronic illnesses
• Health care workers
• Poultry workers
• Long-stay residents of institutions for the disabled
• Individuals living in residential care homes

Individuals outside of these groups may consider vaccination against influenza A. They should be aware of the reasons for giving such vaccination. There is currently no evidence to suggest that influenza A is increasing in incidence. The concern relating to avian influenza might reasonably be expected to reduce the risk of a non avian influenza A epidemic.

What is avian influenza?

Avian influenza is a disease of birds. This disease has been recognized for many years. The current strain of avian influenza is H5N1 which was first identified in 1961. This strain is of concern because it produces severe disease with a high mortality in some domesticated birds including chickens. It has spread geographically in Asia and Europe and is now considered to be endemic.

It is generally very rare for an avian influenza virus to affect humans. However, since December 2003, the World Health Organization (WHO) has reported a total of 121 cases of H5N1 infection. In this group the illness produced severe disease with a high mortality. So far, the majority of infections have occurred in people who have had direct contact with sick birds.

Variation in mortality in infectious disease.

It is important to appreciate that a disease in one population group may act differently in a different situation. The individuals who died from H5N1 had direct contact with very sick or dead birds. This may involve de-feathering or even drinking the blood of an animal with a very high viral load. In many viral illnesses there is a relationship between the size of the genetic exposure (viral load) and the severity of the subsequent disease.

A combination of factors such as viral load, age, nutritional status and individual genetic variables in addition to the tendency for viral illness to attenuate (get less severe over time) mean that the projected mortality of new diseases extrapolated from small cohorts (populations with characteristics different from the whole population) typically exaggerate the severity of a disease within a larger population. Consider the projections for population mortality of HIV in the early 1980s’ or SARS in early 2003.

Is it possible for avian influenza to become more infectious to humans?

It is believed that previous influenza A epidemics occurred when an avian virus developed the capacity to cross the species barrier and spread within the human population. There are 2 possible mechanisms by which the avian influenza virus may make this change.

Antigenic drift: is the term used to describe the slow change over time, which affects all influenza viruses. It is possible that by chance, as the virus changes, it may acquire the ability to spread more easily.

Antigenic shift: This is a sudden change in the structure of a virus. It is possible that influenza A and an avian virus may combine in the same body (human or animal) and swap material, thus producing a hybrid which may spread more easily.

The increasing quantity of circulating avian influenza genetic material in infected birds increases the statistical probability of a mutation by either mechanism occurring. This is the main reason for the concern expressed by the W.H.O.

If the virus changes, will it cause serious disease?

There is no evidence that the virus will cause severe disease if and when it mutates. In the worst case scenario there is a possibility that the virus will cause a severe disease with a high mortality, which spreads easily from person to person. At the other end of the spectrum, it is possible that the virus never mutates or mutates to produce a disease which is either not very infectious or infectious but causes mild symptoms.

In considering the risks of mutation of a new virus there are some important lessons from previous epidemics in addition to basic principles of science and nature:

The vast majority of genetic mutations produce a structure which is not viable.

There have been at least 30 new infectious diseases which have emerged since 1973. In 2003 alone we had SARS, Monkey Pox and West-Nile Disease.

Infectious diseases attenuate (become less infectious) over time.

Projections of mortality extrapolated from one population group early in an epidemic usually greatly exaggerate the severity of the disease.

In the assessment of risk humans tend to amplify virtual risk whilst ignoring real risk.

Prevention of infectious disease

There are two key factors in the prevention of infectious disease:

? Exposure risk

? Host response (Immunity)

Exposure risk:

If you are not exposed to a virus it is not possible to catch the disease. It is sensible at the moment to avoid contact with live bird markets and rural farms or villages in which avian influenza may be endemic.

Enhanced personal hygiene with frequent hand washing significantly reduces exposure risk to droplet borne infections such as influenza.

Host Response:

Individual immunity can be significantly enhanced by leading a healthy lifestyle with a good balanced diet, plenty of exercise, avoidance of smoking and reduction in external stress. During SARS we gave advice on improving immunity which is available on our website www.otandp.com (Password: doctor).

Immunization against infectious diseases also acts by enhancement of specific immunity. Immunization against influenza A is discussed above

What about Tamiflu?

Tamiflu is one medication which has been shown to have some benefit in treatment and prevention of influenza A. The degree of benefit is uncertain and experts disagree about the evidence of effectiveness. It has been suggested that Tamiflu may be effective against an epidemic of avian influenza. Until the virus mutates, it is not possible to produce evidence in favour of this hypothesis.

If an avian influenza virus produces an epidemic of disease within the human population, the mainstay of management is likely to be isolation, quarantine of infected individuals and vaccination of populations once a vaccine is available. It may be possible to use antiviral drugs in both the treatment of infected individuals and protection of at risk contacts. The role of drug treatment in population terms is likely to be very small.

The widespread dissemination of drugs of unproven benefit for a viral mutation that does not yet exist increases the risks of side effects including drug resistance.

How worried should I be?

It is important to understand that all information is filtered by individuals and organizations in different ways.

The World Health Organization has a clear mandate to act on behalf of all of the peoples of the world. Any epidemic of infectious disease is most likely to kill the young, the old and the poor and the W.H.O is undoubtedly acting in the best interests of the world population in getting issues of potential concern to global health at the forefront of the political agenda. The W.H.O produces reputable and reliable information. This information is interpreted by individuals differently. Occasionally W.H.O. representatives contradict the official position of the group. This leads to confusion and uncertainty which in turn increases anxiety.

Assessing this information on an individual basis is very difficult. Information is frequently subject to bias. Bias may be explicit or implicit. Assess all information, including ours, critically and ask if it is logical and rational. Reliable information from a trusted source is critical in situations of change and uncertainty.

Our assessment of the current situation is as follows:-

• There is a potential risk of a mutation occurring, resulting in avian influenza spreading more easily within the human population.
• It is not possible to be sure when, or indeed if, this event will ever happen.
• If such a mutation were to occur it is not possible to be certain whether the disease caused by the resulting virus will be very severe or very mild.
• Whatever the severity of the disease which may be caused by the new virus, it is likely that the disease would become less severe over time.
• Individuals can significantly reduce their risks of infectious disease by reducing exposure risk, especially with hand washing, and improving immunity by healthy lifestyle change.
• High risk groups should receive influenza vaccination. Individuals outside of the high risk groups should make an informed personal decision. Information is filtered by groups and organizations. Much of the information that is received is subject to bias which may be explicit or implicit. It is important to be aware of these biases when assessing the information.
• ndividuals tend to amplify virtual risks and ignore larger present risks.
• Epidemics of anxiety are highly infectious.
• Hong Kong has an excellent public health system.
• It is our opinion that the current risk to our population from the avian influenza virus is extremely small. It is currently many times less than the risks of driving a car. This risk may change over time but it is impossible to predict when and to what extent