We studied the effectiveness of the AS03-adjuvanted monovalent vaccine (Pandemrix(®)) for the prevention of severe pandemic influenza A(H1N1)pdm09 in children, in 2009. All children hospitalized for influenza-like illness in Stockholm County during the peak of the pandemic were included. We compared the frequency of vaccinated children between influenza A(H1N1)pdm09 PCR positive cases and PCR negative controls in a retrospective case-control study. 95 cases and 177 controls were identified. About half of the children in both groups were between 6 months and 2 years of age. Only 1/95 (1%) cases had been vaccinated more than 14 days prior to admission, compared to 23/177 controls (13%), corresponding to a vaccine effectiveness, adjusted for co-morbid conditions, of 91% (95% confidence interval [CI] 30-99). In contrast, the risk for being a case was significantly higher among children vaccinated between 1 and 14 days prior to hospitalization, than among those who were non-vaccinated 13/95 vs. 7/177 (OR 3.6, 95% CI 1.4-9.5). We conclude that a single dose of adjuvanted vaccine was highly protective against hospitalization for influenza A(H1N1)pdm09 in children 6 month to 17 years. The reason for the increased rate of hospitalizations with confirmed influenza in children just following immunization is unclear and should be studied further.
Immunological studies have indicated that the effectiveness of AS03 adjuvanted monovalent influenza A(H1N1)pdm09 vaccine (Pandemrix) may be of longer duration than what is seen for non-adjuvanted seasonal influenza vaccines. Sixty-nine percent of children 6 months-18 years of age in Stockholm County received at least one dose of Pandemrix during the 2009 pandemic. We studied the effectiveness of the vaccine during the influenza seasons 2010-2011 and 2012-2013 in children hospitalized with virologically confirmed influenza. The season 2011-2012 was not included, since influenza A(H3N2) was the predominant circulating strain.
In a retrospective case-control study using a modified test-negative design we compared the percentage vaccinated with Pandemrix among children diagnosed with influenza A(H1N1)pdm09 (cases), with that of those diagnosed with influenza A(H3N2) or influenza B (controls) during the two seasons. We excluded children born after July 1, 2009, since only children who were 6 months of age or older received the pandemic vaccine in October-December 2009.
During the 2010-2011 season, 3/16 (19%) of children diagnosed with influenza A(H1N1)pdm09, vs. 32/41 (78%) of those with influenza A(H3N2) or influenza B had been vaccinated with Pandemrix in 2009. The odds ratio, after adjustment for sex, age and underlying diseases, for becoming a case when vaccinated with Pandemrix was 0.083 (95%CI 0.014, 0.36), corresponding to a VE of 91.7%. During the season 2012-2013, there was no difference between the two groups; 59% of children diagnosed with influenza A(H3N2)/B and 60% of those with influenza A(H1N1)pdm09 had been vaccinated with Pandemrix in 2009.
The AS03 adjuvanted monovalent influenza A(H1N1) pdm09 vaccine (Pandemrix) was effective in preventing hospital admission for influenza A(H1N1)pdm09 in children during at least two seasons.