Vaccination strategies and critical vaccination coverage
Deterministic modelling approach
The equilibrium of a system differential equations can be used to estimate the critical vaccination coverage which prevents circulation and successful reintroduction of wild polio virus.
- Critical vaccination coverage for OPV vaccination
If the orally administered life vaccine (OPV) is used, it has to be taken into account that the vaccine virus itself (although it is no longer pathogenic) causes an infection and can spread to contect persons who also will become immunized. In a population where OPV is used, the wild polio virus has to compete with the vaccine virus for susceptible individuals. The spread of the vaccine virus thereby further reduces the critical vaccination coverage.
- Critical vaccination coverage for IPV vaccination
In contrast, the inactivated vaccine (IPV) consists of virus particles which cannot cause an infection of the vaccinee. This has the advantage that the vaccination risik is largely reduced. As OPV can (in very rare cases) cause severe clinical symptoms, many industrialized countries have recently shifted their vaccination policy from the use of OPV to that of IPV. IPV is highly potent in preventing clinical disease, but does not always protect against carriage of the wild polio virus, which increases the critical vaccination coverage. If wild virus transmission is favored by low hygienical standards and population crowding, a situation can occur where even a vaccination coverage of 100 % is no longer sufficient to eliminate wild virus transmission using IPV.
Stochastic modelling approach
- The phenomenon of local extinction
The transmission of an infection within a small population can cease simply because of stochastic effects, even without vaccination. In order to establisch endemic transmission, the population must exceed a critical size (see critical population size). The probability of extinction further increases with vaccination.
- Vaccination days with OPV
A few years ago, some tropical countries started implementing so-called national vaccination days. On each of these vaccination days, children are given the opportunity to receive OPV. According to simulation studies, an annual vaccination coverage as low as 20-25 % may be sufficient to obtain a stable wild virus-free population.