Public Release:  Could vaccination for children against seasonal flu stop immunity developing against pandemic strains?

Lancet

Infection with "seasonal" influenza A could actually benefit children by giving them improved immunity against pandemic strains such as the swine flu H1N1 strain now circulating. Thus, vaccinating children aged 6 months to 5 years against seasonal flu, as is recommended in some countries, could be disadvantageous to these children. This controversial theory is discussed in a Personal View published Online First and in the December edition of The Lancet Infectious Diseases, written by Dr Guus Rimmelzwaan, Erasmus Medical Center, Rotterdam, Netherlands, and colleagues. In an accompanying comment, Drs Terho Heikkinen and Ville Peltola, Turku University Hospital, Turku, Finland, disagree, saying that preventing children catching seasonal flu is a definite benefit of vaccination which far outweighs the theoretical risk put forward by Rimmelzwaan and colleagues.

It has been shown, mainly in animals, that infection with influenza A viruses can induce protective immunity to influenza A viruses of other unrelated subtypes. Rimmelzwaan and colleagues say: "This so-called heterosubtypic immunity does not provide full protection, but can limit virus replication and reduce morbidity and mortality of the host."

They add that this type of immunity might be relevant to human beings when a new subtype of influenza A virus is introduced into the population, such as the new influenza A H1N1 virus (swine flu) responsible for the current pandemic; and also potential future pandemics that may be caused by avian flu subtypes. While avian flu in humans is still very rare, it does cause high levels of mortality in those infected. The authors say: "Preventing infection with seasonal influenza viruses by vaccination might prevent the induction of heterosubtypic immunity to pandemic strains, which might be a disadvantage to immunologically naive people, such as infants."

The present pandemic could be a unique opportunity to investigate heterosubtypic immunity. The authors say that close monitoring of admissions to hospital and mortality rates among infants that have received annual influenza vaccination since birth and comparison with unvaccinated age-matched children might provide information on the potential downside of yearly influenza vaccination.

Rimmelzwaan and colleagues conclude: "The development and use of vaccines that can induce broad protective immunity might be a solution for these potential problems and we think this is a priority."

Importantly, Rimmelzwaan and colleagues add that they fully support the forthcoming vaccination programme against H1N1 influenza, which will reduce severe disease and mortality in all age groups.

They say*: "Use of these pandemic influenza vaccines will over-ride the theoretical issues associated with yearly vaccination against seasonal influenza."

In the linked Reflection and Reaction comment, Heikkinen and Peltola say that "the results of experimental animal studies can never be extrapolated directly to human beings, let alone form the basis of any vaccination policy".

They do agree with Rimmelzwaan and colleagues on one point- the need for more effective influenza vaccines that would also induce broader immune responses.

However, they conclude: "Public health decisions should be based on the best clinical evidence available. There is ample evidence for the great burden of influenza in young children, and this burden appears during every influenza season. By contrast, there is no clinical evidence that vaccinating children against influenza would prevent the induction of heterosubtypic immunity and thereby be disadvantageous to children in the long run. While waiting for improved influenza vaccines, the simple question is should we let young children suffer from a severe and potentially lethal but easily preventable illness, just because there is a theoretical possibility that withholding vaccination might result in a slightly less severe illness sometime in the future? We believe that the answer to this question is a simple one."

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Dr Guus Rimmelzwaan, Erasmus Medical Center, Rotterdam, Netherlands. T) +31-10-7044066 . E) g.rimmelzwaan@erasmusmc.nl

Ville Peltola, Turku University Hospital, Turku, Finland. E) ville.petola@utu.fi

For full Personal View and Reflection and Reaction, see: http://press.thelancet.com/tlidchildvacc.pdf

*Note to editors: quote direct from Dr Rimmelzwaan and cannot be found in text of Personal View

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