An influenza pandemic is a global outbreak

An influenza pandemic is an almost simultaneous outbreak of influenza in all areas of the world. An influenza pandemic is caused by an influenza virus that is new to people. Therefore, almost nobody has a sufficient immunologic defence against it. This is different from the normal annual influenza outbreaks. Although the ordinary influenza virus changes slightly from year to year, healthy people still have acquired partial protection against it by previous contacts with the influenza virus or vaccination. During annual influenza outbreaks the main concerns are the risk of a severe course of influenza in young children or in the elderly and in people with reduced immunity.

In a pandemic one can speak less about special risk groups for a serious course of the disease because nobody is immune. Historically, about every 25-50 years an influenza pandemic has occurred. In the last century this has happened three times: in 1918 (Spanish influenza), 1957 (Asian influenza) and 1968 (Hong Kong influenza). The severity varied with an estimated 1-50 million excess deaths during the pandemics. The time and place of the next pandemic are impossible to forecast. However, in 2009 the WHO declared a pandemic with A(H1N1) virus from swine origin, which causes in general mild influenza (WHO, 2009a).

Conditions for a pandemic influenza outbreak

WHO has defined the characteristics of a new influenza virus that can cause a pandemic (WHO, 2009b) as follows:

• A new influenza virus subtype emerges
• It infects humans, causing serious illness
• It spreads easily and sustainable among humans.

The WHO global influenza preparedness plan has defined 6 phases of an influenza outbreak in which phase 6 defines the stage of a pandemic outbreak (WHO, 2009c).

The A (H1N1) virus source

The present influenza A(H1N1) virus is a new virus subtype of influenza affecting humans, which contains segments of genes from pig, bird and human influenza viruses in a combination that has never been observed before anywhere in the world. New viruses are often the result of a re-assortment of genes from two other viruses (swap of genes). This A(H1N1) virus is the result of a combination of two swine influenza viruses that contained genes of avian and human origin.

Typical swine influenza is an acute viral infection of the respiratory tract in pigs, caused by type A influenza virus. The mortality rate in pigs is low and recovery usually occurs within 7–10 days. Swine-origin influenza virus infections also occur in wild birds, poultry, horses and humans, but interspecies transmission is considered a rare event. So far three influenza type A virus subtypes have been found in pigs: H1N1, H1N2 and H3N2. Human infections with swine influenza have been detected occasionally since the late 1950s usually in persons with direct exposure to pigs (e.g. people working in pig farms, etc.). In Europe, since 1958 a total of 17 cases have been reported (ECDC, 2009a).

First appearance in Mexico and the USA

Novel influenza A(H1N1) was first detected in Mexico and the United States in March and April, 2009 (CDC, 2009b).

Similar transmission routes as normal influenza

People become infected with Influenza A(H1N1) virus in the same way as for normal seasonal influenza. It spreads from person to person via droplets from an infected person who is coughing and sneezing; indirectly when droplets or secretions from the nose and throat settle on objects (including hands) which then are touched by other people who touch their own mouth or nose (ECDC, 2009a).

The usual influenza symptoms

Symptoms of influenza A(H1N1) virus in humans are usually similar to regular human seasonal influenza symptoms, involving fever of sudden onset and respiratory symptoms; diarrhoea might also occur. However, severe complications could occur even in normally healthy persons who become infected with the virus (ECDC, 2009a). At this time, most people who have become ill with novel A(H1N1) in the United States have recovered without requiring medical treatment and have experienced typical influenza symptoms (CDC, 2009c).

Increasing severity according to WHO’s pandemic alert system

WHO uses a series of six phases of pandemic alert to inform the world of the seriousness of the threat of an influenza outbreak and the need to launch progressively more intense preparedness activities (WHO, 2009c).

Based on the assessment of all available information and following several expert consultations on influenza A(H1N1), on 11 June 2009 WHO had raised the existing level of influenza pandemic alert to phase 6, i.e. to the pandemic stage. At that moment sustained community-level transmission of the virus was taking place in more than one region of the world. The pandemic was characterized as being of moderate severity. Most of the cases had been mild, but a large spread of mild cases together with even a low fraction of severe cases can have a very significant impact on global health systems and society (WHO, 2009d).