The suggestion that yearly immunization against influenza might make children more susceptible to serious disease during a pandemic has generated some controversy. Does this idea have merit?
If you have read Being older is a good defense against 2009 H1N1 influenza, you are familiar with the concept of ‘heterosubtypic immunity’. After natural infection with influenza virus, the host produces T and B cells directed against internal proteins of the virions. These viral proteins are more conserved among different strains than the surface glycoproteins HA and NA. Upon infection with a different subtype – which occurs during a pandemic – heterosubtypic immunity could limit virus replication and reduce disease and death.
Evidence for heterosubtypic immunity to influenza virus comes mainly from studies in guinea pigs and mice. It has been suggested that this type of immunity could explain why older people appear to be less susceptible to infection with the 2009 H1N1 influenza virus.
What does heterosubtypic immunity have to do with vaccinating children against influenza? Immunization of children against influenza is a good idea, because it reduces the amount of disease. But immunized children don’t develop heterosubtypic immunity – it’s not induced by the vaccine, and natural infection is prevented. This could put children at greater risk for more serious disease when pandemic strains emerge. Absence of heterosubtypic immunity might explain why young children are more likely to develop severe disease after infection with the 2009 H1N1 influenza virus.
Is there a solution to this potential problem? One possibility is more widespread use of infectious, attenuated influenza vaccines such as Flumist, which are known to induce heterosubtypic immunity. New vaccines that induce heterosubtypic immunity are under development. These consist of purified internal virion proteins, such as M2 protein, that undergo less antigenic variation than HA or NA.
Should we stop immunizing young children against influenza, and wait for the development of better vaccines? Of course not! Influenza in children can be serious. We should not allow this preventable disease to occur based on an unproven theory that children might be better off in a future pandemic. Neither do the authors of the hypothesis advocate cessation of immunization:
More research is needed to find out if heterosubtypic immunity contributes to protection against infection with pandemic strains in people and if yearly vaccination against seasonal influenza prevents the induction of heterosubtypic immunity. The development and use of vaccines that can induce broad protective immunity might be a solution for these potential problems and we think this is a priority.
The results of animal experiments do not dictate immunization policy. But what we learn from animal experiments often lead to hypothesis which can be tested by studies in humans.
Bodewes R, Kreijtz JH, & Rimmelzwaan GF (2009). Yearly influenza vaccinations: a double-edged sword? The Lancet infectious diseases PMID: 19879807
Heikkinen T, & Peltola V (2009). Influenza vaccination of children. The Lancet infectious diseases PMID: 19879806