By Bruce Y. Lee
The U.S. is pinning its hopes on a COVID-19 coronavirus vaccine, but will a vaccine alone be enough to stop the pandemic and allow life to return to normal?
The answer depends on a how “good” the vaccine ends up being.
In a study published July 15 in the American Journal of Preventive Medicine , my colleagues and I used a computer simulation of every person in the country to show how effective a vaccine would have to be and how many people would have to get vaccinated to end the pandemic. We found that a coronavirus vaccine’s effectiveness may have to be higher than 70% or even 80% before Americans can safely stop relying social distancing. By comparison, the measles vaccine has an efficacy of 95%-98%, and the flu vaccine is 20%-60%.
That doesn’t mean a vaccine that offers less protection would be useless, but it would mean social distancing in some form may still be necessary.
What is vaccine ‘effectiveness’ anyway?
Some political leaders have suggested that society will return to normal soon, especially if a vaccine becomes available by the end of the year or early in 2021. Some vaccines are currently in early-stage trials, but that timeline would still be very optimistic.
However, it is important to remember that a vaccine is like many other products: What matters is not just that the product is available but also how effective it is. Take clothing for example. If you are going to a formal dinner, underwear alone may partially cover you but probably not well enough for the occasion. This doesn’t mean underwear is useless.
Similarly, different vaccines may offer different levels of protection. Scientists talk about this as the vaccine’s efficacy or effectiveness. If 100 people who haven’t been exposed to the virus are given a vaccine that has an efficacy of 80%, that means that on average 80 of them would not get infected.
The difference between efficacy and effectiveness is that the former applies when vaccination is given under controlled circumstances, like a clinical trial, and the latter is under “real-world” conditions. Typically, a vaccine’s effectiveness tends to be lower than its efficacy.
Computer simulations show what could happen
Since COVID-19 coronavirus vaccines are still under development, now is the time to set vaccine efficacy levels to aim for, as well as to manage expectations. Running computer simulations is really the only way to ethically do this.
For the study, our PHICOR team at the City University of New York Graduate School of Public Health and Health Policy , working with scientists from the National School of Tropical Medicine at the Baylor College of Medicine , developed a computer simulation model of the entire United States and its population interacting with each other. Using that model, we were able to introduce the COVID-19 virus into this virtual population in different ways and have it spread from person to person in various pandemic scenarios. Each simulated person who gets infected has probabilities of being hospitalized, placed on a ventilator or dying based on the severity of the problems just as in the real world.
Experiments using this simulated population can represent the different vaccines and what is likely to happen if different proportions of the population are vaccinated at different times during the pandemic. The results show how vaccines with different levels of efficacy would affect the pandemic and can be used to estimate the impact on things such as number of people who get infected, health outcomes and costs. In this case, we assumed that only one vaccination would be required.