Impact of the introduction of A/Sydney/5/97 H3N2 influenza virus into South Africa.

In 1998 South Africa experienced a major influenza epidemic that was characterized by extensive illness and an unusually early season. The impact of the epidemic was charted by measuring proxy indexes of influenza activity such as school absenteeism and excess mortality in persons older than 65 years. Viruses isolated from patients of all age groups were analyzed both antigenically and at the molecular level to determine the characteristics of the influenza strain responsible for the outbreaks. The study revealed that influenza activity was detected as early as the middle of April and peaked toward the end of May and early June. School absenteeism correlated with a sharp rise in virus isolation during this period. Consumption of influenza-related pharmaceuticals, as well as mortality figures, also corresponded to the increased absenteeism and virus isolation. Characterization of the viruses isolated during 1997 and 1998 showed clearly that the epidemic was caused by the introduction of the A/Sydney/5/97-like H3N2 influenza strain into South Africa in 1998. With no prior exposure to this virus strain, which is antigenically distinct from the viruses that had been present in this country in 1997, the population was highly susceptible, resulting in an early, rapid spread of influenza. This epidemic has highlighted the importance of having an influenza vaccine specifically formulated for the Southern Hemisphere. If the 1998 vaccine had not contained the A/Sydney/5/97 strain, the widespread outbreaks in South Africa would have been far worse in terms of morbidity, mortality, and economic loss. This, in turn, emphasizes the need for increased influenza surveillance and international cooperation. Copyright 1999 Wiley-Liss, Inc.