By Pedro Mendes, University of Connecticut
It has been clear for some time that at least in the US, the only way out of the coronavirus pandemic will be vaccination. The rapid deployment of coronavirus vaccines is underway, but how many people will need to be vaccinated to control this pandemic?
I am a computational biologist who uses data and computer models to answer biological questions at the University of Connecticut. I tracked my state’s COVID-19 epidemic using a computer model to predict the number of hospital stays at the University of Connecticut’s John Dempsey Hospital.
This type of computer model and the theory behind it can also be used to calculate the vaccination rates required to break the coronavirus transmission chain. I estimate that around 70% of the population across the US will need to be vaccinated to stop the pandemic. Differences in people’s behavior in different parts of the country, as well as unanswered questions about whether the vaccine completely prevents infection or just prevents people from getting sick, create a degree of uncertainty.
Switch off the transmission
Clinical studies have shown that once a person has been vaccinated against the coronavirus, they will not get COVID-19. A person who does not get sick can still be infected with the coronavirus. But let’s also assume that a vaccinated person cannot pass the virus on to others, although the researchers still don’t know if this is the case.
When enough of the population is vaccinated, the virus will find it difficult to find new people to become infected and the epidemic will begin to die out. And not everyone needs to be vaccinated, just enough people to keep the virus from spreading out of control. The number of people who need to be vaccinated is called the critical vaccination level. Once a population reaches this number, you will be given herd immunity. Herd immunity is when there are so many people vaccinated that it is difficult for an infected person to find someone to become infected, and hence the virus cannot spread to other people. This is very important to protect people who cannot be vaccinated.
The critical level of vaccination depends on how contagious the disease is and how effective the vaccine is. Infectivity is measured using the base reproduction number R0. This is the number of people an infected person would, on average, spread the virus to if no protective measures were taken.
The more contagious a disease is, the more people need to be vaccinated in order to achieve a heard immunity. The more effective the vaccine, the fewer people need to be vaccinated.
Not the same everywhere
The R0 values differ from place to place because their populations behave differently. Social interactions are not the same in rural and urban areas, or in warm climates compared to cold ones.
Using the data on positive cases, hospitalizations, and deaths, my model estimates that Connecticut currently has an R0 of 2.88, meaning that, on average, each infected person would pass the virus on to 2.88 other people unless mitigation measures were taken would. The county-level estimates range from 1.44 in rural Alpine, California to 4.31 in urban Hudson, New Jersey.
However, the determination of an R0 value for the entire United States is particularly difficult due to the different climatic zones and the fact that the virus affected different areas at different times – the behavior was anything but uniform. Estimates vary between 2.47 and 8.2, although most researchers put R0 at around 3 for the entire United States.
While R0 varies by location and between estimates, the effectiveness of the vaccines is constant and known. The vaccines Pfizer-BioNTech and Moderna prevent 95% and 94.5% COVID-19, respectively.
Using the values for the effectiveness of the vaccine and the R0, we can calculate the critical degree of vaccination. Connecticut, with an R0 of 2.88, requires 69% of the population to be vaccinated. For the entire USA with an R0 of 3, this would be 70%. In New York City this would be 80% with an estimated R0 of 4.26.
While the math is relatively simple, things get complicated when you look at key questions that epidemiologists don’t yet have answers to.
First, the critical vaccination level formula assumes that humans interact randomly. In the real world, however, people interact in highly structured networks, depending on work, travel and social connections. When these contact patterns are taken into account, some researchers found that critical vaccination levels are significantly lower compared to assuming random interactions.
Unfortunately, other unknowns could have the opposite effect.
Vaccine studies clearly show that vaccinated people will not get COVID-19. However, it is not yet known if the vaccines will prevent people from getting mild infections that they could pass on to others. If vaccinated people are still infected and able to pass the virus on, vaccination does not provide herd immunity – although it would prevent serious diseases and drastically reduce mortality.
One final question that remains to be answered is how long does immunity to the coronavirus last after a person is vaccinated. If the immunity wears off after a few months, each individual must be vaccinated repeatedly.
It’s hard to say for sure how many people will need to be vaccinated to end this pandemic. Even so, the introduction of COVID-19 vaccines was the best news in 2020. In 2021, as a large proportion of the people in the US receive the vaccine, the country will be heading for critical vaccination levels – whatever it may be – so that life can get back to normal.
[You need to understand the coronavirus pandemic, and we can help. Read The Conversation’s newsletter.]
Pedro Mendes, Professor of Cell Biology, University of Connecticut
This article is republished by The Conversation under a Creative Commons license. Read the original article.