Several inactivated mono vaccines against the H1N1v flu virus have been licensed, or will be by the end of 2009. They differ in several ways, such as the type of vaccine (whole virus, split, or subunit), whether or not they contain a lipid adjuvant (to boost the immune response and also to facilitate industrial production), single or multi-dose presentation, whether or not they contain a preservative.

Drug regulatory authorities have put in place accelerated procedures to evaluate the data supplied by pharmaceutical companies and to allow the rapid commercialisation of H1N1v flu vaccines. As at 30 September 2009, only a few preliminary results of short-term immunogenicity studies of people in good health had been published.

Split virus and subunit H1N1v flu vaccines with no adjuvants are similar to standard seasonal flu vaccines. According to preliminary results of one study, the immune response was considered satisfactory in over 75% of 240 adults under the age of 65 who had been given an H1N1v vaccine of this type. In preliminary results of another study of 70 children who had been given another H1N1v vaccine of the same type, the response was, as forecast, not as good among the youngest.

A seasonal flu vaccine with lipid adjuvant MF59C.1 has been on the market for several years. In one study, the response of over 75% of 100 adults aged under 50 who were given an H1N1v vaccine of this type was considered satisfactory. The lipid adjuvant AS03 has a similar composition to that of MF59C.1, suggesting that it has a comparable immunogenic effect. As at 30 September 2009, no evaluation data on the whole virus H1N1v vaccine, the only flu vaccine of this type, had been published.

In immunogenicity studies on adults, the adverse effects were as predicted (mainly local reactions and muscle pain). Adverse effects were more frequent with lipid adjuvants. The adjuvant MF59C.1 only rarely exposes patients to severe systemic adverse effects, and has been around longer than AS03, allowing better evaluation of its long-term effects.

In infants and pregnant women, the presence of a lipid adjuvant in H1N1v flu vaccines raises immunological questions, and is possibly a cause of febrile convulsions in infants.

Active monitoring of adverse neurological effects, such as Guillain-Barré syndrome, is required as for all other flu vaccines, and especially with the whole virus vaccine.

A single-dose presentation with a pre-filled, ready-to-use syringe is the best option for protecting against bacterial contamination and risks associated with a preservative.

In practice, although still fragmentary, the data available as at 30 September 2009 points to the advisability of vaccinating people at a high risk of severe complications from H1N1v flu, and also those around them and who care for them. Several vaccines are available. The vaccine chosen, subject to availability, should be one with a minimum risk of adverse effects, especially in infants and pregnant women considered to be at risk: in other words preferably a split virus vaccine with no adjuvants.

For more on the H1N1 flu:

> More on the H1N1 flu: the truth about antivirals, and the risks for pregnant women and infants, December 2009
> Flu prevention: first and foremost, wash hands often, December 2009

©Prescrire December 2009

Source: "Vaccins grippaux A/H1N1v (Celvapan°, Focetria°, Pandemrix°, Panenza°)" Rev Prescrire 2009 ; 29 (313) ; 806-810.