The H1N1 influenza strain Mexico 2009 (H1N1pandemic09) led to mild symptoms (with no or low fever) in Estonia during the 2009–2010 outbreak. Due to the lack of clinical signs, it was difficult to estimate the real spreading of this influenza virus in Estonia and no cases of H1N1 influenza were officially registered in animals either. We used an ELISA method to screen blood sample collections for the presence of anti-H1N1 and anti-H3N2 antibodies. All sera were also tested with the hemagglutination inhibition (HI) assay. Out of the 123 samples from human patients, 23 (i.e. 18.7%) were seropositive for the H1N1pandemic09 virus. In addition, blood samples from six persons were positive for both H1N1 and H3N2 viruses, while according to the data from the Estonian Health Board, people aged 15–65 had a general disease rate of around 3.9%. Almost all of the tested animals from two herds (out of four studied) were seropositive for H1N1pandemic09. The seven HA protein sequences isolated from Estonia were aligned with a consensus sequence of the pandemic H1N1 HA sequences from Mexico using ClustalW, and 12 amino acids substitutions were found.
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