What the extent of B-type mismatch between the influenza types covered by the trivalent influenza vaccination and actual strains of infecti…

Table 1 summarises the influenza B vaccine strains and circulating influenza B virus types for each of the previous three seasons: 2011-2012, 2012-2013 and 2013-2014. Information has been provided for all ages due to the small numbers of samples in two out of three seasons. Caution is required when interpreting these results due to the low level of influenza B detected and hence samples processed in 2011-12 and 2013-14. Please note that 2012-13 was characterised by a higher than usual influenza B activity and a decision was made to type all influenza B positive samples.
Due to the small number of samples tested for influenza B type in the 2011-12 and 2013-14 seasons, it is not possible to provide figures broken down by health board to avoid deductive disclosure. Whilst the number of samples typed for influenza B for the 2012-13 season are greater we are only able to provide a breakdown by circulating vaccine strain by region due to small numbers of samples tested in some boards (table 2)
Table 1: Flu B vaccine and circulating strains for the last three influenza seasons in Scotland
Season
Flu B vaccine strain (trivalent)
Circulating flu B strains (% total)
Comment regarding sample size
2011-12
Victoria
Victoria (59%), Yamagata (41%)
Based on a small number of samples (n=17), prior to introduction of routine lineage testing for all flu B isolates
2012-13
Yamagata
Victoria (22%), Yamagata (78%)
Based on a larger number of samples (n=667)
2013-14
Yamagata
Victoria (30%), Yamagata (70%)
Based on a small number of samples (n=10)

Table 2: Regional breakdown of circulating Influenza B types in Scotland in 2012-13
Region
Yamagata
Victoria)
All
West
285 (76%)
89 (24%)
374
East
71 (76%)
23 (24%)
94
North
67 (86%)
11 (14%)
78
Total
423 (78%)
123 (22%)
546*
* Whilst the number of samples available for analysis from season 2012-13 was 667 amplification to allow lineage was only possible on 546 of these samples

Unlike influenza A viruses, there are only two lineages of influenza B virus, Yamagata and Victoria. Due to an increasing number of influenza B circulating strains being of the Yamagata lineage (type) in recent years, the World Health organization (WHO) changed the influenza B vaccine strain component of the annual trivalent seasonal influenza vaccine from Victoria to Yamagata in 2012-13.
A ‘mismatch’ is where the flu viruses in the vaccine are significantly different from those circulating in the community. In years when the vaccine strains are not well matched to circulating strains, vaccine effectiveness may be reduced. However, even when the viruses in the vaccine and circulating viruses are not well matched, a vaccine may still offer some protection against circulating viruses. For example, B/Yamagata/Wisconsin strain in the 2012-13 trivalent vaccine provided some protection against B/Yamagata/Massachussetts (contained in the 2013-14 vaccine). However, B/Yamagata strains will not provide protection against B/Victoria. Vaccines containing one influenza B lineage would not be expected to give a significant level of protection against the other lineage. Hence the WHO recommendation for development of quadrivalent vaccines with both B lineages.