The results of a follow-up study looking at the persistence of antibody response following the administration of pandemic influenza vaccine to children in the UK, and the effects of a further dose of trivalent vaccine, have been published by the NIHR Health Technology Assessment programme.

The authors note that the pandemic influenza vaccine was offered to children in the UK during the 2009-2010 influenza season.  There was however some debate as to whether these children needed to be revaccinated during the following year, when the pandemic virus was still circulating.  They therefore conducted a follow-up of a previous clinical trial comparing two pandemic influenza vaccines in children, to determine the persistence of antibody and response to the 2010-11 trivalent seasonal influenza vaccine.

In the original study, 943 children aged 6-12 months received two doses of either 1) a non-adjuvanted whole virion H1N1 influenza vaccine; or 2) an adjuvanted split-virion H1N1 influenza vaccine.  Around a third of these children (n=323) were enrolled into this follow-up study, which involved having a blood sample taken one year later (to assess antibody persistence), the administration of one dose of the trivalent 2010-11 seasonal influenza vaccine, and then a further blood sample three weeks later.  A microneutralisation (MN) titre ≥ 1:40, or a haemagglutination (HI) titre ≥ 1:32, was considered to be indicative of serological protection against disease.

The main results reported are as follows:
    

• For those who received the whole-virion vaccine, a MN titre of ≥ 1:40 was seen after one year in 32.4% of those who were vaccinated when they were <3 years old and in 65.9% of those vaccinated at ≥3 years.  HI titres of ≥ 1:32 were seen in 63.2% and 79.1%, respectively.

• For those who received the adjuvanted vaccine, a MN titre of ≥ 1:40 was seen in 100% and 96.9%, and a HI titre of ≥ 1:32 in 98.4% and 96.9%, respectively.

• One year after either pandemic vaccine, the 2010–11 trivalent seasonal influenza vaccine produced a marked serological response to the H1N1 component of the vaccine - all patients available for analysis had a MN titre of ≥ 1:40 and a HI titre of ≥ 1:32. 

• The trivalent vaccine was well tolerated, although in children aged <5 years, a fever was reported in 13.6% of those who had previously received the whole-virion vaccine and in 18.3% of those who received the adjuvant vaccine.

The authors comment that children given monovalent pandemic influenza vaccines still had protective antibody titres one year later; persistence beyond this time remains unknown.  Administration of the trivalent vaccine boosted the antibody titre and was well tolerated.  They say that they will look to investigate whether or not previous receipt of monovalent influenza vaccines affected serological response to the H3N2 and B components of the 2010–11 seasonal influenza vaccine, using stored sera.