Vaccine Efficacy Required to Prevent or Extinguish COVID-19 Epidemic

Results from simulation experiments published in the American Journal of Preventative Medicine¹ found that in order for a vaccine to extinguish the ongoing coronavirus disease 2019 (COVID-19) pandemic in the United States without any additional measures, the vaccine must have an efficacy of 80%. To prevent an epidemic the simulations determined vaccine efficacy must reach at least 70%.

Currently, the implementation of social distancing remains one of the few measures available to address the COVID-19 pandemic. The push for a vaccine that will eliminate the need for additional measures like social distancing continues. To determine the level of efficacy these vaccines will require, investigators used a previously developed computational transmission, clinical, and economics outcomes model representing the United States population (n=357,157,434) and their different interactions with each other, as well as the spread of SARS-CoV-2 and the potential health and economic outcomes.²

The simulation experiments demonstrated that in order to prevent an epidemic — defined as reducing the peak by >99% — vaccine efficacy needs to be at least 60% when coverage is 100% with R₀ equals 2.5 to 3.5. The efficacy threshold rises to 70% when coverage drops to 75%. If coverage drops to 60% with R₀ equal to 2.5, vaccine efficacy of up to 80% is required. When coverage drops to 75% and R₀ is 3.5, simulations found that vaccine efficacy of 80% is needed to prevent an epidemic.

In order to extinguish an ongoing epidemic, vaccine efficacy has to be at least 60% when coverage is 100%, and at least 80% when coverage drops to 75%. This would reduce the peak by 85% to 86%, 61% to 62%, and 32% when vaccination occurs after COVID-19 exposure in 5%, 15%, and 30% of the population, respectively.

“A vaccine with an efficacy between 60% and 80% could still obviate the need for other measures under certain circumstances such as much higher, and in some cases, potentially unachievable, vaccination coverages,” the researchers wrote.

Investigators acknowledge and remind readers that all models, by definition, are simplifications of real-life scenarios and do not account for every possible scenario. The model inputs that were used drew from various sources, and new SARS-CoV-2 data is continually emerging. Therefore, the actual course of the epidemic may not be predictable. To address this, investigators assessed a range of possible scenarios and parameter values in their sensitivity analyses.

Specific limitations include the mixing of individuals in a way that is not likely representative of a real world situation, as well as the possibility that targeted vaccine approaches may have higher impacts. Also, the experiments assumed that vaccinations would occur in a single day, when in reality this would take an extended period of time, and the effect of vaccination would be reduced. The models also made assumptions about levels of herd immunity required, which may, in fact, depend on differing individual susceptibility to SARS-CoV-2 as well as the availability of sufficient healthcare resources for all patients. If the latter is not true and the healthcare system is overburdened, patients may not receive proper care, resulting in higher mortality.

“These efficacy and coverage thresholds hold as the proportion of the population exposed increases from 5% to 30%, at which point the peak of the epidemic is rapidly approached as more and more people become exposed and immune,” the researchers concluded. “Achieving…75% coverage is not trivial.”

References

Bartsch SM, O’Shea KJ, Ferguson MC, et al.  Vaccine efficacy needed for a COVID-19 coronavirus vaccine to prevent or stop an epidemic as the sole intervention. [published online July 15 2020]. Am. J. Prev. Med. doi: 10.1016/j.amepre.2020.06.011