The United States has pinned its hopes on finding a vaccine against COVID-19, but would a vaccine alone be enough to slow the pandemic and allow life to return to normal?
The answer depends on how good that vaccine turns out to be.
In an article published July 15 in the American Journal of Preventive Medicine , my colleagues and I used a computer simulation based on the population of the United States to see how effective a vaccine should be, and how many. people would have to be vaccinated to end the pandemic.
We found that the efficacy of the coronavirus vaccine would have to be greater than 70%, and even as high as 80%, for Americans to be able to safely move past current social distancing measures . By comparison, the measles vaccine is 95% to 98% effective. The flu, between 20% and 60%.
This is not to say that a vaccine that offers less protection is not helpful, but that certain social distancing measures will still be necessary.
What is the “efficacy” of a vaccine?
Some political leaders have stated that countries will soon return to normality, especially if a vaccine is available by the end of the year or early 2021. Although it is true that some vaccines are currently in the preliminary trial phase, these deadlines they are very optimistic.
However, it is important to remember that a vaccine is like many other products: the important thing is not only to be able to have them, but also to what extent they are effective. Take clothing as an example. If you go to a formal dinner, underwear covers your body only partially, much less than the situation would require … But that does not mean that it is useless.
Similarly, different vaccines could offer different levels of protection. Scientists refer to this as either the efficacy or the effectiveness of a vaccine.
If we give 100 people who have not been infected a vaccine whose efficacy is 80%, that means that, on average, 80 of them will not be infected.
The difference between efficacy and effectiveness is that the former refers to when the vaccine is applied under well-controlled conditions, such as a clinical trial, while the latter is used when it is given under real conditions.
Typically, the effectiveness of a vaccine is less than its effectiveness.
Simulations show what could happen
As coronavirus vaccines are still in development, at this time we have to set our targets in terms of efficacy and know how to manage expectations. The only way to do both ethically is through computer simulations.
For this study, our PHICOR project team at the City University of New York, in collaboration with scientists from the National School of Tropical Medicine at Baylor School of Medicine (both in the USA, have developed a model of computer simulation that reproduces the social interactions of the population of the United States as a whole.
Thanks to this model, we were able to simulate various levels of contagion among this virtual population and also to project how its expansion would be in various pandemic scenarios. In each of them the chances were calculated that an infected person would end up needing hospitalization, require a respirator or die. All based on the severity of their pathologies, just as it happens in the real world.
Experiments using this model can project the effects of different vaccines. And also what could happen if during the duration of the pandemic only a fraction of the population received it.
The results show the extent to which vaccines with different levels of efficacy would have an impact on the pandemic. In addition, these conclusions could also be used to calculate the impact of the vaccine on aspects such as the number of people who would be infected and the costs and results of health policies. In this case, we take it for granted that only one vaccine would be necessary.
What will it take to stop the pandemic?
Under normal conditions, both in epidemic and pandemic situations, and the more people are exposed to the virus, the number of newly infected daily increases steadily until it reaches a peak and begins to decline. But, of course, the time that this process can be delayed depends on how both the virus itself and the ways to combat it evolve.
To stop the pandemic it is necessary that the number of newly infected drops to zero (or at least a very low number) as soon as possible.
If the COVID-19 pandemic were starting and the percentage of the infected population was close to 0%, the simulations show that the efficacy of the vaccine should be at least 60% to stop the coronavirus, and the entire population should be vaccinated .
This scenario is probably not possible since, in the first place, there is a percentage of people who could not be vaccinated due to age or health problems. And second, many people would refuse to be given the vaccine.
If only 75% of the population were vaccinated, the efficacy of the vaccine would have to be around 70%. And if only 60% of the vaccine were given, the efficacy threshold would have to be even higher, around 80%. The key is to make sure the chain of virus contagion is slowed.
These figures are based on the premise that each infected person infects an average of 2.5 more people. Thus, if the virus were to become more contagious, the vaccine would have to be more efficient as well.
However, the longer the pandemic lasts, the less room there will be to reduce its incidence compared to its maximum level. It is like when you go up a mountain, which already starts from a certain height. In addition, it is more difficult to end a pandemic when there is an increasing number of people who can infect it.
Thus, when around 5% of the population is already infected with the virus, the best that can be done is to reduce the incidence of the virus by around 85% compared to its peak level. The difference between 0% and 5% of infected implies millions of infections. At this point, there is evidence that around 1% of the United States population is infected, although there are government sources who believe that the true percentage is much higher.
The percentage of people who get vaccinated will be key
With all of the above in mind, a vaccine as low as 60% efficacy could serve to slow the pandemic and allow society to return to normal. However, for this to be the case, most, if not all, of the world’s population would have to be vaccinated.
This seems unlikely when we consider that surveys indicating that only three-quarters of Americans say they would agree to receive a coronavirus vaccine if they were assured that it was safe.
With fewer people protected, the vaccine would have to be at least 80% effective in order to stop the pandemic on its own, that is, completely eliminating social distancing measures. This data could serve as a reference to set a goal for the development of different vaccines against coronavirus.
Again, this is not to say that a vaccine with a lower efficacy should be considered useless. It would only mean that social distancing measures and the obligation to wear a mask would continue until the pandemic was subsiding naturally or until a good enough vaccine appeared.
Bruce Y. Lee. Professor of Health Policy and Management, City University of New York
This article was originally published on The Conversation. Read the original.
