A computer model was made where the underlying theory can also be used to calculate the vaccination rates needed to break the chain of coronavirus transmission. The rough estimate is that across the United States, as an example, approximately 70% of the population needs to be vaccinated to stop the pandemic. But variation in how people behave in different parts of the world, as well as open questions about whether the vaccine prevents infection altogether or simply keeps people from getting sick, add a degree of uncertainty.
Clinical trials have shown that once a person gets vaccinated for the coronavirus, they won’t get sick with COVID-19. A person who doesn’t get sick can still be infected with the coronavirus. But let’s also assume that a vaccinated person can’t spread the virus to others, though researchers still don’t know if this is true. When enough of the population is vaccinated, the virus has a hard time finding new people to infect, and the epidemic starts dying out. And not everyone needs to be vaccinated, just enough people to stop the virus from spreading out of control. The number of people who need to be vaccinated is known as the critical vaccination level. Once a population reaches that number, you get herd immunity. Herd immunity is when there are so many vaccinated people that an infected person can hardly find anyone who could get infected, and so the virus cannot propagate to other people. This is very important to protect people who cannot get vaccinated. The critical vaccination level depends on how infectious the disease is and how effective the vaccine is. Infectiousness is measured using the basic reproduction number – R0 – which is how many people an infected person would spread the virus to on average if no protective measures were in place. The more infectious a disease is, the larger the number of people who need to be vaccinated to reach heard immunity. The higher the effectiveness of the vaccine, the fewer people need to be vaccinated.
While the math is relatively simple, things get complicated when you consider important questions for which epidemiologists still have no answers. First, the formula for critical vaccination level assumes that people interact randomly. But in the real world, people interact in highly structured networks depending on work, travel and social connections. When those contact patterns are considered, some researchers found critical vaccination levels to be considerably smaller compared to assuming random interactions. Unfortunately, other unknowns could have an opposite effect. Vaccine trials clearly show that vaccinated people don’t get sick with COVID-19. But it is still unknown whether the vaccines prevent people from getting mild infections that they could pass on to others. If vaccinated people can still be infected and pass on the virus, then vaccination will not provide herd immunity – though it would still prevent serious disease and reduce mortality drastically. A final question that remains to be answered is how long immunity to the coronavirus lasts after a person is vaccinated. If immunity wanes after a few months, then each individual will need repeated vaccinations. It is hard to say with certainty how many people need to be vaccinated in order to end this pandemic. But even so, the arrival of COVID-19 vaccines has been the best news in 2021.
Awesome and detailed description. #Neha Mittal and Peer's Team.