Miller E, Hoschler K, Hardelid P, Stanford E, Andrews N, Zambon M Incidence of 2009 pandemic influenza A H1N1 infection in England: a cross-sectional serological study The Lancet, Early Online Publication, 21 January 2010 doi:10.1016/S0140-6736(09)62126-7 http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(09)62126-7/fulltext
This detailed paper describes how unlinked-anonymised residual blood samples from a regular annual seroepidemiological monitoring programme were utilized to describe the background immunity to the 2009 pandemic virus in the community in the main part of the UK (England). The same surveillance was then used to monitor the impact of the Spring and Summer wave of the pandemic.
Pre-existing immunity was common in older people but very uncommon in the young. When the Spring Summer wave took place its impact was non-uniform especially affecting two regions including the UK capital (London). The proportion of children infected in the higher incidence areas was found to be about one in three, around ten times higher during the Spring / Summer wave than would have been suggested by clinical surveillance. The finding also indicated how children had an important role in the transmission of this pandemic virus and would be a key target group for vaccination.
ECDC comment (January 22st 2010):
Though not the first published seroepidemiological report from Europe (1) this is the most detailed so far. It is accompanied by a commentary paper from the United States CDC (2) and a UK explanation describing how this resource intensive work (expensive and labour intensive serological tests are required) was supported financially with managed emergency funding.(3)
It is important not to assume that these initial findings from the UK will necessarily translate to the rest of Europe. The Spring/Summer wave in the UK was unusual in Europe in that it was so large, stopped in July and then resumed in the Autumn.(4) The results from clinical surveillance shown by the European Influenza Surveillance Network suggest the Autumn/Winter waves in most European countries were more uniform than those experienced in the Spring and Summer in the UK where infection rates may also have been affected by vigorous delaying activities.(4).
However it will be important to continue this kind of work both in the UK and in other European countries. ECDC will be supporting and coordinating such seroepidemiological work, along with WHO though the main funding would need to come from countries themselves.(3) It is only by such studies that Europe will know what proportion of the population remain vulnerable and therefore how likely further waves are to take place. Though the rates of the pandemic virus infection are now declining across Europe there is a lot more we need to learn about this new influenza as it becomes part of the seasonal pattern.(5)
The CDC Commentary describes how the background immunity was similar to that observed in North America. It is also fitting with what was observed clinically (there were few older patients affected in Europe). However, older people should not be neglected for immunization since though they had low infection rates they experienced the highest mortality rates of those infected of any age-group.(6) Intriguing findings are mentioned in the in CDC commentary since it seems that findings from serosurveys or serological studies in China and Japan are not the same as in the US and UK. Few older people in China possessed antibodies to the 2009 pandemic while in Japan only very old people (born before 1920) had antibodies cross-active to the pandemic virus.(2)
1. INVS 2009b SéroGrippeHebdo seroprévalence du virus A (H1N1) 2009 chez les femmes enceintes. Bulletin Grippe A(H1N1) 2009 No. 75. Point de situation au 15 décembre 2009 INVS December 15th 2009.
2. Reed C, Katz J Serological surveys for 2009 pandemic influenza A H1N1 http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(09)62194-2/fulltext
3. Walley T, Davison P. Research funding in a pandemic http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(10)60068-2/fulltext
4. HPA. Pandemic (H1N1) 2009 in England: an overview of initial epidemiological findings and implications for the second wave. 2 December 2009. http://www.hpa.org.uk/web/HPAwebFile/HPAweb_C/1258560552857
5. ECDC Risk Assessment ECDC 2009 Risk Assessment 2009 influenza A(H1N1) pandemic Version 7 – 17 December 2009
6. Nicoll A. A new decade, a new seasonal influenza: the Council of the European Union Recommendation on seasonal influenza vaccination Eurosurveillance January 7th 2010 http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19458