Chances of COVID eradication ‘pretty slim,’ Yale researcher says
Would a vaccine be a panacea, stopping the coronavirus in its tracks? If a vaccine were to be approved for distribution today, would things go back to the way they were?
“It's not necessarily going to make everything go away so that it's never a problem,” said Virginia Pitzer. “The chances of that happening at this point, that we eliminate and eradicate the coronavirus altogether, are pretty slim. It's going to take massive effort to do that, given the fact that it's now globally present in countries around the world.”
Piitzer is an associate professor of the epidemiology of microbial disease at the Yale School of Public Health. As she put it, “I use mathematical and statistical models to try and better understand the transmission dynamics of infectious diseases, working mostly on vaccine-preventable diseases.”
In general, to have a significant effect on disease transmission, to reach the “herd immunity threshold” — the point at which enough people have immunity to the virus that it cannot spread — a vaccine would need to have 67 percent efficacy, Pitzer said.
That means the vaccine would need to be 67 percent effective at conferring immunity and administered to 100 percent of the population, or be 100 percent effective and administered to 67 percent of the population, or somewhere in between.
“The idea there is that if 67 percent of the population is immunized against the virus, then on average one case can only infect one or fewer other cases within the population and the disease is inevitably going to die,” Pitzer said.
That doesn’t mean a vaccine shouldn’t be a goal. Pitzer said a vaccine with lower efficacy than that 67 percent threshold would still be valuable.
“That doesn't mean that a less effective vaccine is not of any use to the population. There certainly would be value in a less effective vaccine as well,” she said. “Some fraction of the population — particularly in Connecticut where we had a relatively severe first wave of the infection — some fraction of the population is already immune. And so the reality is that, in order to see cases start to decline now, you could vaccinate or have a vaccine that's less than 67 percent effective and you'd see start cases start to decline because you already have some immunity from natural infection.”
And that 67 percent is just a baseline. There are still many caveats that need to be considered.
For example, a potential vaccine would need to be effective against both symptomatic and asymptomatic infections, Pitzer said.
And then, of course, it is not known yet how long immunity against COVID-19 might last. Consider the flu. That virus mutates so quickly that you have to get another flu shot every season.
The flu differs from COVID in that it is far less transmissible, meaning you don’t need to immunize as high a population against the virus every season.
“You would only need to effectively immunize 50 percent of the population to eliminate flu,” Pitzer said. “However, that only pertains to a particular strain of flu. Because flu is constantly mutating to escape both natural immunity as well as vaccine induced immunity, by the time you reach that immunity threshold for flu, it's already moved on to being a different strain.”
As for how long immunity might last, Pitzer said other coronaviruses offer a clue.
“The expectation based on our understanding of other coronaviruses is that there will be some immunity lasting at least on the order of nine months or more, but exactly how we measure that immunity,” and how that immunity would be conferred by a vaccine, is still being studied, Pitzer said. “I think scientists are really just trying to understand those things now.”
There are a few vaccine candidates being considered, but Pitzer pointed to one in particular.
“The one that is kind of farthest along when it comes to the development process and testing process is the vaccine that's coming out of Oxford,” she said. “They've already started the phase three trials, which are the big trials where they're actually looking for the efficacy of the vaccine against the symptoms of COVID.”
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