Mathematical modeling of the effects of vaccine efficacy and the speed of vaccination show that a lower efficacy vaccine may curb disease transmission similar to that of a higher efficacy vaccine if the speed of immunization is high. A new study published on a preprint server analyzed vaccine efficacy and speed of vaccine distribution to understand how these affect controlling the virus’s spread with the emergence of new variants.
The authors used the susceptible-infected-recovered-deceased model (SIR-D), a mathematical model used for simulating infectious diseases, to determine how vaccines with different efficacies affect disease spread and how vaccine distribution speed affects the infection attack rate (IAR). In their simulation, the team assumed that all vaccines required only a single dose and the person becomes immunized immediately.
The team found that without any vaccines, the IAR is about 81%. When the virus mutation time is five days, at 500,000 doses per day, the IAR decreases to 72-75%. When the number of doses per day is increased three times, the IAR drops to about 60-65%. The analysis showed that even a vaccine with low efficacy initially, which they assumed to be around 65%, whose efficacy reduces to about 60% after the new variants, can have a low IAR compared to a high efficacy vaccine (95%) if the number of doses administered is high.
If the virus mutates before the daily infection peak, the highest fraction of the population who get newly infected on a single day, then a moderate efficacy (75%) vaccine works best. Once the peak daily infections are crossed, the number of daily infections drops faster when a moderate efficacy vaccine is administered. Increasing the speed of distribution of high efficacy mRNA vaccines may be difficult, given its stringent storage requirements, so it may be more advantageous to use resources to distribute a lower efficacy vaccine faster, as this would quickly reduce the susceptible population.
Thus, the model shows that the speed of vaccine distribution plays a more important role than vaccine efficacy in containing the pandemic, emphasizing the need for wide and rapid vaccine coverage.