title: Pandemic influenza A(H1N1)pdm09 seroprevalence in Sweden before and after the pandemic and the vaccination campaign in 2009.
abstract: The immunity to pandemic influenza A(H1N1)pdm09 in Sweden before and after the outbreaks in 2009 and 2010 was investigated in a seroepidemiological study. Serum samples were collected at four time points: during 2007 (n = 1968), in October 2009 (n = 2218), in May 2010 (n = 2638) and in May 2011 (n = 2513) and were tested for hemagglutination inhibition (HI) antibodies. In 2007, 4.9% of the population had pre-existing HI titres ≥40, with the highest prevalence (20.0%) in 15-24 year-olds, followed by ≥80 year-olds (9.3%). The overall prevalence of HI titres ≥40 had not changed significantly in October 2009. In May 2010 the prevalence had increased to 48.6% with the highest percentages in 5-14 year-olds (76.2%) andlowest in 75-79 year-olds (18.3%). One year later the prevalence of HI titres ≥40 had increased further to 52.2%. Children 5-14 years had the highest incidence of infection and vaccine uptake as well as the highest post-pandemic protective antibody levels. In contrast, the elderly had high vaccine uptake and low attack rate but low levels of protective antibodies, underlining that factors other than HI antibodies are involved in protection against influenza A(H1N1)pdm09. However, for all age-groups the seroprevalence was stable or increasing between 2010 and 2011, indicating that both vaccine- and infection-induced antibodies were long-lived.
authors.affiliation: Department of Diagnostics and Vaccinology, Swedish Institute for Communicable Disease Control, Solna, Sweden. andreas.morner@smi.se
authors.forename: Andreas
authors.initials: A
authors.lastname: Mörner
authors.forename: Andreas
authors.initials: A
authors.lastname: Bråve
authors.forename: Anna-Maria
authors.initials: AM
authors.lastname: Kling
authors.forename: Sharon
authors.initials: S
authors.lastname: Kühlmann-Berenzon
authors.forename: Katarina
authors.initials: K
authors.lastname: Krook
authors.forename: Mona
authors.initials: M
authors.lastname: Hedenskog
authors.forename: Irene
authors.initials: I
authors.lastname: Silhammar
authors.forename: Margaretha
authors.initials: M
authors.lastname: Ljungman
authors.forename: Ake
authors.initials: A
authors.lastname: Ortqvist
authors.forename: Sören
authors.initials: S
authors.lastname: Andersson
authors.forename: Maria
authors.initials: M
authors.lastname: Brytting
authors.forename: Rigmor
authors.initials: R
authors.lastname: Thorstensson
authors.forename: Annika
authors.initials: A
authors.lastname: Linde
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D007251:Influenza, Human / Q000097:blood / Q000453:epidemiology* / Q000276:immunology; D008297:Male; D032541:Mass Vaccination; D008875:Middle Aged; D058873:Pandemics / Q000706:statistics & numerical data; D016036:Seroepidemiologic Studies; D013548:Sweden / Q000453:epidemiology; D055815:Young Adult