The best way to protect yourself against flu is to get a single shot of influenza vaccine each year before the cold weather season starts, which is September and October north of the equator or April/May in the southern hemisphere. The WHO strongly recommends the use of vaccine as an effective preventive measure against this potentially fatal disease. Vaccination has been associated with a reduction in influenza-associated and all-cause mortality of 30-50% or more. Studies conducted in elderly persons from the US, Canada, the UK, Spain and Argentina have shown that influenza vaccination is also associated with a reduction in complications of influenza, including decreases in hospitalization by 19-57% for pneumonia, 17-39% for all respiratory conditions, and up to 38% for congestive heart failure.
Rapid structural changes in the most commonly circulated types of influenza virus necessitate annual changes in the composition of the vaccine. Scientists focus on detecting which types of viruses are in circulation 9 or 10 months before the flu season begins in order to produce an effective vaccine.
Currently there are strains of three different influenza (sub)types circulating worldwide in humans; two subtypes of influenza A, H1N1 and H3N2, and one of influenza B. Influenza vaccines are therefore trivalent and contain two A strains, subtypes H1N1 and H3N2, and one B strain. Due to the frequent mutations of these viruses the World Health Organization (WHO) maintains a worldwide surveillance of influenza viruses and gives annual recommendations on the strains to be included in the vaccine.
The vaccine composition for the season 2005/2006 (northern hemisphere) contains the following three components:
- an A/New Caledonia/20/99(H1N1)-like virus
- an A/California/7/2004 (H3N2)-like virus
- a B/Shanghai/361/2002-like virus