According to the results of a Danish cohort study published early online in the British Medical Journal (BMJ), the adjuvanted monovalent vaccine offered in the 2009/10 influenza season offered protection against laboratory-confirmed H1N1 infection, but did not have any significant effect on H1N1-related hospital admissions in patients with underlying chronic diseases.  

The authors note that several vaccines against H1N1 were developed rapidly and licensed based on approval of mock-up vaccines, following the worldwide spread of this novel type of influenza in 2009.  Several countries set up observational studies to determine clinical effectiveness of these vaccines, and the results of these suggested estimates of effectiveness in the general population of 72-90%.  Estimates of vaccine effectiveness in patients with underlying conditions (a target group) have however been limited. 

The authors of the current study used data from mandatory national reporting systems in Denmark to determine the effectiveness of an adjuvanted monovalent vaccine (Pandemrix®) among all Danish people <65 years of age with underlying chronic diseases (n=388,069).  The main outcome measures included laboratory confirmed H1N1 infection and influenza related hospital admission with laboratory confirmed H1N1 infection. Estimates of vaccine effectiveness were adjusted for age and underlying disease.

The main findings reported were as follows:

• A total of 799 people in the cohort tested positive for H1N1 infection, of whom 718 were not vaccinated with the pandemic vaccine, 49 were vaccinated 1-7 days before the date of swabbing, 18 were vaccinated 8-14 days before the date of swabbing, and 14 were vaccinated more than 14 days before the date of swabbing.

• The effectiveness of pandemic vaccine against confirmed H1N1 infection 14 days after one dose of vaccine was 49% (95% CI 10% to 71%).  No significant effectiveness was seen when the vaccine was given less than 14 days before the date of swabbing.

• Among the 229 patients that were admitted to hospital, 188 were not vaccinated with the pandemic vaccine, 25 were vaccinated 1-7 days before admission, eight were vaccinated 8-14 days before admission, and eight were admitted more than 14 days after vaccination. 

• No significant effectiveness of the pandemic vaccine against admission to hospital for confirmed H1N1 was observed 14 days or more following vaccination (44%; 95% CI −19% to 73%).

The authors discuss the strengths and limitations of their study, and comment that their results are of public health relevance, as those with underlying chronic illnesses are a major target group for influenza immunisation and because of the likely delayed availability of pandemic vaccines in a forthcoming pandemic.