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.
To combat the COVID-19 pandemic, several vaccines have now been approved and are being administered to populations across the globe. However, one concern has been the vaccines' efficacy against emerging variants of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). These vaccines were only tested on the original strain of the virus.
Using a vaccine with high efficacy against circulating variants is useful to achieve herd immunity quickly. However, with lower vaccine efficacy in the face of newer variants or if vaccines cannot be administered quickly, it is challenging to choose which vaccine should be favored.
The mRNA vaccines from Moderna and Pfizer have an efficacy of about 94-95% and have to be stored in a freezer at extremely low temperatures. The Johnson and Johnson single-dose vaccine has an efficacy of about 66% and can be stored in the refrigerator. Although several vaccines have been approved, their distribution and administration have been slow.
Three main new SARS-CoV-2 variants have been identified so far. Most vaccines have similar efficacy against the B.1.1.7 strain (United Kingdom) but have decreased efficacy against the B.1.351 strain (South Africa).
Modeling infection spread and vaccination speed
In a paper published on the medRxiv* preprint server, researchers 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.
But, if the virus mutation occurs after the daily peak infection, a vaccine with high efficacy before the new variants emerged and a low efficacy after, gives a lower IAR at all dosage rates.
Fast vaccination rate is key
Hence, a vaccine with low efficacy before and after the new variants arose and can still give a low IAR if administered rapidly, compared to a high efficacy vaccine with slow deployment. Even if a high efficacy vaccine has reduced efficacy to new variants, it will still lower the IAR if used for rapid immunization.
There has been a shortage of vaccines around the world despite manufactures ramping up production. mRNA vaccines require ultra-cold freezers for storage, reducing the number of administration sites. If administration sites do not have such storage facilities, these vaccines need to be thrown away, leading to wastage.
In contrast, adenovirus-based vaccines can be stored at the same temperature as the flu vaccine. They can use the same vaccine distribution chain, leading to less wastage and more administration sites. So, although the Johnson & Johnson single-dose vaccine has a lower efficiency than the mRNA vaccine, it can be deployed faster, which may be beneficial despite its lower efficacy.
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.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.