Effectiveness of a MF-59™-adjuvanted pandemic influenza vaccine to prevent 2009 A/H1N1 influenza-related hospitalisation; a matched case-control study

Table 2 Effectiveness of the pandemic influenza vaccine in preventing 2009 influenza A/H1N1-related hospitalisation.

	VE (%) (95%CI)
	Exposed if > 7 days between vaccination and symptom onset	Exposed if > 14 days between vaccination and symptom onset
VE*	19 (-28-49)	< 0 (upper 95%CI 30)
VE imputed data^$	24 (-113-73)	26 (-160-79)
Restricted VE	49 (16-69)	35 (-26-66)
Restr. VE imputed^$	51 (-43-83)	59 (-65-90)
Maximum VE^#	74 (53-86)	61 (19-82)

The most realistic estimate of the vaccine effectiveness (VE), the VE estimation whereby we restricted our controls to those suffering from severe underlying medical conditions (restricted VE) and the maximum VE are presented. Results derived using different cut offs to define exposure are presented using the assumption based on cases with known date of vaccination and using imputed data.
* For 36 cases date of vaccination was missing. Respectively 49% and 31% of the vaccinated cases with unknown vaccination date were assumed to be exposed using 7 days or 14 days as cut off for the validity of vaccination (extrapolation from cases with known date of vaccination).
^$ We used multiple imputation and sampled (n = 10) from a uniform distribution with a lower bound of delay = 0 and an upper bound of delay = number of days since the start of the study + 7 days. This upper bound is based on the start of the vaccination campaign relative to the start of this study.
^# For the maximum VE, all vaccinees with unknown vaccination date were assumed unexposed.

ISSN: 1471-2334