HI antibodies predict protection. Partial protection between variant viruses of the same subtype. No protection between subtypes,. And so, as with FMDV, annual vaccines have to match up (by HI tests and sequencing of the neutralisation sites on H) as well as possible with the 2 or 3 subtypes the animal is most likely might meet during the next year.

Early hopes that a Tc cell vaccine would protect against all members of a subtype of even type have not been realised. This hope is now being applied by the ex-MRC flu immunologists, now at Oxford, to a new HIV vaccine. This idea was prompted by the discovery that some active Kenyan sex workers had developed Tc to HIV instead of AIDS.