1015 Safety of H1N1 vaccines

In light of the pandemic nature of the H1N1 virus in 2008/9 and its significant morbidity and mortality, vaccine development was swift and vaccination programmes were rolled out worldwide. South Africa, as host of the 2010 FIFA World Cup during its winter season, is potentially at risk as travelers descend on the country from areas which include several that have already been affected. Immunisation, particularly of high-risk groups is being promoted and the usual questions are being asked about safety of the vaccine. Memories of the increased risk of Guillain-Barre Syndrome (GBS) following immunization against swine ‘flu in 1976/7 are often at the heart of the adverse-affect debate, although several studies have shown that subsequent versions of the vaccine during more-recent epidemics have not been associated with GBS. Importantly, as pointed out in 2009 by Black et al, with potentially hundreds of millions vaccines, events will inevitably occur in some who were vaccinated in the preceding period, and the tendency will be to relate the apparently-adverse event to the vaccine. For this reason it is imperative to have background rates of such events so as to assess whether there is in fact a causal relationship. On the basis of reviewed data in the UK, if 10 million individuals were to be vaccinated there would be a background rate of 21.5 cases of GBS and 5.75 sudden deaths within 6 weeks of vaccination (i.e. for an event to be causally-related these rates would have to be significantly higher in vaccines). In female vaccines in the USA there would be an expectation of 86.3 cases of optic neuritis per 10 million within 6 weeks, and 397 per 1 million pregnant women would be expected to have a spontaneous abortion within 1 day of vaccination. A report from Taiwan, based on 2004-8 data, looked also at other events that have been raised as possible consequences of influenza vaccine; these include GBS, optic neuritis, Bell’s palsy, anaphylaxis, seizures and spontaneous abortion. Some of these background rates are fairly high e.g. Bell’s palsy in 106 per 100 000 patient years for people aged >18 years, and around 105 per 100 000 patient years would be expected to have a seizure.

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Lancet 2010; 375: 1146-7 and 1164
Lancet 2009; DOI:10.1016/S0140-6736(09)61877-8
New Engl J Med 1998; 339: 1797-802