Among the many spectacular promises that Donald Trump made when accepting the Republican nomination for the US Presidency, the boldest was his oft-repeated claim that “We will have a safe and effective vaccine this year.”

A number of researchers, including Australian scientists, are already claiming they have a vaccine that offers protection against the coronavirus.

But the real questions are: how effective does a vaccine need to be –and how many people will need to be vaccinated – in order for life to return to normal?

“Normal” means a life in which there is no need for for masks, social distancing and limits on gatherings.

The less people jabbed, the better the vaccine needs to be

In a new US study, a computer simulation model found that if 75 per cent of the population gets vaccinated, the vaccine has to have an efficacy (ability to protect against infection) of at least 70 per cent to prevent an epidemic and at least 80 percent to extinguish an ongoing epidemic.

Researchers have calculated how well a vaccine needs to work against the coronavirus to restore life to normal. Photo: Getty

If only 60 percent of the population gets vaccinated, the thresholds are even higher, around 80 percent to prevent an epidemic and 100 percent to extinguish an ongoing epidemic.

Lead investigator, Bruce Y. Lee, Professor of Health Policy and Management, City University of New York, explained it this way in a Conversation piece:

“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 per cent, that means that on average 80 of them would not get infected,” Professor Lee explained.

“The difference between efficacy and effectiveness is that (efficacy) applies when vaccination is given under controlled circumstances, like a clinical trial, and (effectiveness is what’s measured) under ‘real-world’ conditions.

“Typically, a vaccine’s effectiveness tends to be lower than its efficacy.”

Professor Lee said that a coronavirus vaccine’s effectiveness may have to be higher than 70 per cent or even 80 per cent before Americans can safely stop relying social distancing.

By comparison, he said, the measles vaccine has an efficacy of 95-98 per cent, and the flu vaccine is 20-60 per cent.

The US population as a model

Professor spoke in terms of the US population, because the computer simulation model was of the entire United States “and its population interacting with each other”.

Using that model, he said, “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.”

The value of the modelling for public health policy

Professor Lee said 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.”

The results could help shape expectations for policy makers, business leaders, and the general public.

He said the modelling was based on the assumption that only one vaccination would be required.

“Some are pushing for a vaccine to come out as quickly as possible so that life can ‘return to normal,'” said Professor Lee.

“However, we have to set appropriate expectations. Just because a vaccine comes out doesn’t mean you can go back to life as it was before the pandemic.”