The recent publication of a supplement issue of EIMC dedicated to review the Spanish experience during the influenza pandemic declared in year 20091 offers an useful source of data for sounding properly a debate in regard to the suitability of the criteria formulated by the corresponding World Health Organisation (WHO) ad hoc expert committee for warning about such pandemics at present and in the future. In contrast with the position sustained by WHO until the beginning of year 2009, the new criteria introduced at that time allow the declaration of an influenza pandemic for the case of a type A viral strain from a subtype already circulating in the past, provided it becomes worldwide distributed at a given time and shows a series of special features in regard to antigenicity either fully demonstrated or just predicted by genetic studies. The first influenza pandemic declared in the 21st century met these new criteria, although not the former ones.
From the excellent epidemiological analysis offered in the issue for Spain,2 this new pandemic displayed three main, significant features: (1) the epidemic wave started earlier in time than in seasonal influenza; (2) the pandemic strain substituted to almost one hundred per cent the seasonal strains circulating during the previous years; and (3) the incidence found among the older than 65 years was very significantly lesser than in seasonal influenza. The first one was, however, not totally unique, since it had been already observed to a lesser extent during influenza seasons 1996–1997 and 2003–2004 over the last 14 ones. Unlike in the prior pandemics resulting from the emergence of subtypes H2N1 and H3N2, the second one did not last in time, even in the short time span of two seasons.3
As stated by the authors, the last feature reflected the existence of efficient protective immunity against the pandemic strain among that population group. Immune people would have most likely been those who experienced the first contact in life with influenza A virus before 1957, when just the H1N1 subtype was circulating among humans. This would support again the well-known concept of “antigenic original sin” of influenza. In addition, as less elder people became infected because of pre-existing immunity, the involvement of the youngest increased in percentage terms when analysing both morbidity and mortality. This effect limits the meaning of interpretations favouring a higher pathogenic potential of the pandemic strain for children and young adults. Actually, comparison of age-specific case fatality rates rather than population mortality rates in the Spanish experience did not support such assertions.2 Considerations about immunopathogenesis, risk factors for serious illness, and ability to produce pneumonia4–7 for the pandemic strain are also of great interest, but contribute very few to examine its potentially differential features because of the lack of useful data from seasonal virus strains for a proper comparison.
The experience teaches that, whatever the agent involved could be, declaring a pandemic of an infectious disease is much more than a technical issue. Once the message reaches the politicians and the population – and it happens nowadays extremely fast and in a largely uncontrolled fashion – the social consequences multiply and may be serious. The declaration of an influenza pandemic should involve, in the opinion of many, some consideration of seriousness for the population. The mere analysis of the geographical distribution of the virus in time, or the predictions about differential antigenic properties drawn from genetic studies, cannot be enough. Since the 2009 influenza pandemic has come to be unanimously qualified as mild in comparison with prior pandemics, but also with some seasonal epidemics, it can be concluded that the first challenge that the new WHO criteria for declaring influenza pandemics had to meet did not support its adaptation to the real needs of the citizens and of the society. In many aspects, this last pandemic displayed many of the characteristics leading formerly to qualify the influenza epidemics of 1947 and 1977 as pseudopandemics.8 Such a denomination would perhaps be much more adequate than the word pandemic for the behaviour shown by the A(H1N1)pdm09 strain and for the epidemic it rose up.
A return to the concept that influenza pandemics must only be declared after the emergence of type A virus strains belonging to subtypes novel to the human beings seems, in conclusion, worth to be considered and discussed.
