We develop a transmission model to examine three facets of prophylactic vaccine failure: take (the vaccine may only work in a fraction of those who are vaccinated), degree (the vaccine may only reduce and not eliminate the probability of infection upon exposure), and duration (the vaccine may only confer protection for a limited time period).

We demonstrate:

1. how to derive a summary measure of vaccine imperfection;

2. how to calculate the critical vaccination coverage that is required to eradicate an HIV epidemic;

3. how to assess the potential impact of different types of imperfect prophylactic vaccines (i.e. vaccines that fail in different way) in both clinical trials and mass vaccination campaigns.

We present analytic and scenario results, the latter being based upon parameter values that are derived from the HIV epidemic in gay men in San Francisco, California. For further details see McLean and Blower (1993).

References

McLean, A.R., and Blower, S.M. (1993) ‘Imperfect vaccines and herd immunity to HIV’, Proc. Roy. Soc. Land. B253, 9.

Discussion

Garnett To what extent do the results of Blower and McLean depend on their assumption of an exponential decay in loss of vaccine-induced immunity?

Reply The initial HIV vaccine model that we have presented is based upon reasonable biological assumptions and is also based upon what is known about the mechanism of action of prophylactic vaccines for other diseases. The data that are available from (non-HIV) clinical trials with long follow-up periods, although fairly limited, suggest generally that vaccine-induced immunity is either life-long or tends to decay exponentially.