INFLUENZA IN THE WORLD

Prof. Lam Sai Kit, Director, WHO National Influenza Centre, Department of Medical Microbiology, Faculty of Medicine, University of Malaya, Kuala Lumpur.

In the last few weeks, there has been increasing concern locally to the increasing trend of severe influenza around the world, particularly in the Northern Hemisphere. This release from the WHO National Influenza Centre in Malaysia is to provide up-to-date information of the actual situation of influenza in the world.

The 1997-1998 influenza season has been classified as moderate to severe by the WHO in the Northern Hemisphere, with epidemics in many countries, while activity was less severe in the Southern Hemisphere. Canada and the United States of America reported marked increase, with regional to widespread activity in mid-January. During the last influenza season (October 1998-February 1999), influenza occurred sporadically in many countries in the Northern Hemisphere with outbreaks reported in Africa, Asia, Europe and North America.

Explosive outbreaks during the winter months

Influenza virus is transmitted from person to person in virus-laden droplets expelled during coughing and sneezing. Within a few hours of infection, the virus begins to replicate at a very high rate throughout the respiratory tract, reaching a peak after about 2 days. The short incubation period, together with high viral titres in secretions, the lengthy period of viral shedding and the relatively small amounts of virus necessary to initiate infection, explain the explosive nature of outbreaks of influenza in the community during the winter months.

Influenza is characterized by abrupt onset of various respiratory and systematic symptoms, such as fever, cough, sore throat, coryza, fatigue, headache and muscle aches. Most complications associated with influenza infection are respiratory (bronchitis, croup, pneumonia, bronchiolitis, lung abscess and asthma), but infection can also affect many major organs, such as muscles, brain and heart. Primary viral or secondary bacterial pneumonia is a particularly serious complication in the elderly and the cause of death within 2 days of admission.

Influenza strains

Both influenza types A and B were common during the outbreaks in the last 2 seasons in the Northern and Southern Hemispheres. Influenza A (H3N2) predominated in some countries while influenza B viruses predominated in others. There were very few strains of influenza A (H1N1) reported. Influenza B was particularly active in Belgium, Italy, Netherlands, Spain and Switzerland.

 

 

Antigenic characteristics

The majority of the influenza A (H3N2) isolates was antigenically closely related to A/Sydney/5/97. However, there were some strains that were sufficiently different from A/Sydney strain.

Influenza virus isolates were antigenically related to B/Beijing/184/93 and close antigenic variants continued to cocirculate in Asia.

Recommendations for the composition of influenza virus vaccines

During the 1998-1999 season, influenza A(H3N2), influenza A(H1N1) and influenza B viruses continued to circulate, with influenza A(H3N2) and influenza B predominating. Based on antigenic analyses by WHO, the following vaccine composition is recommended for the Northern Hemisphere:

The majority of the influenza A(H3N2) isolates in the Southern Hemisphere was antigenically related to A/Sydney. However, an increasing proportion of viruses was antigenically distinguishable from A/Sydney and more closely related to A/Moscow/10/99. The majority of the influenza A(H1N1) viruses were related antigenically most closely related to A/Caledonia/20/99.

The following vaccine composition for the Southern Hemisphere in the coming season (May-October 2000) is recommended:

Meeting the challenge of influenza

The annual impact of influenza, both on the individual and on society as a whole, has been greatly underestimated. Very few countries have looked into the economic impact of the disease. In the Northern Hemisphere, 100 million people are infected with influenza virus each year and most of these patients are debilitated in some way. In addition, the number of deaths as a result of influenza is considerable, particularly for patients in high-risk groups. In patients with cardiovascular and pulmonary disease, the death toll during epidemics from influenza or pneumonia can be as high as 870 per 100,000 population. The costs associated with influenza include direct healthcare costs, indirect costs such as lost productivity, and intangible cost of pain, grief and social disruption. Estimates of lost productivity in Germany in 1995/96 were over US$900 million.

Control of Influenza

Control of influenza is almost exclusively based on the use of vaccines. Vaccination is particularly recommended for those at high risk of developing severe influenza or its complications, such as individuals with chronic respiratory disease, chronic heart or kidney disease, diabetes, immunocompromised individuals and those aged over 65. Annual vaccination has been shown to be effective in preventing influenza and its complications and is cost-effective.

One dose of the vaccine in those who had previous exposure to influenza or previously immunized should be immunogenic for individuals of all ages except children. Previously unimmunized children should receive 2 doses of vaccine with an interval between doses of at least 4 weeks.

It must be remembered that vaccination does not offer 100% protection and in older persons, protection is about 70% on the average.

Role of antivirals

A number of antivirals against influenza are available. Amantadine and rimantadine have been in use for many years and can be used for either prevention or treatment. They are only effective against influenza A infection and are associated with the rapid development of resistance and toxicity.

The latest antiviral to hit the market is a neuraminidase inhibitor (Zanamivir). Neuraminidase is a protein on the surface of the influenza virus and essential for viral replication in the body and highly conserved amongst influenza viruses, both A and B.

When given by inhalation, it has proven efficacy in treating naturally acquired influenza in adults. It has to be administered via the inhaled and/or intranasal route because of its poor oral bioavailability and may be used in the early treatment of influenza. Individuals who have not been vaccinated before the influenza season and have possibly been exposed during epidemics will benefit from early treatment because of its relatively fast action.

It must be emphasized that antivirals are not recommended to replace vaccination, which will undoubtedly remain the major cornerstone in the battle against influenza.

Influenza in Malaysia

The WHO National Influenza Centre in the University Hospital is one of 110 national centres in 83 countries and plays an integral role in the monitoring of influenza viruses around the world. Representative strains isolated by the UH Influenza Centre are sent for antigenic analyses to WHO and the information contributes towards the formulation of the next season’s influenza vaccine composition.

Attached are tables showing the types of respiratory viruses isolated in 1998 and 1999. Of the 24 influenza strains isolated in 1998, 23 (95.8%) were influenza A. In 1999, 34 influenza strains were isolated of which 28 (82.4%) were influenza A. The number of influenza B isolates has increased significantly from 1 in 1998 to 6 isolates in 1999. In the first week of 2000 alone, we have isolated 3 strains of influenza B and no influenza A.

In younger children, especially those under 2 years, respiratory syncytial virus (RSV) and parainfluenza viruses are the most important cause of lower respiratory infection, not influenza. Paediatricians should be alerted to this situation in the country.

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