Why A COVID-19 Vaccine That's Only 50% Effective Could Still Help Stop The Pandemic : Shots - Health News - NPR

"When we talk 'vaccine effectiveness,' what we're talking about is, 'How effective was the vaccine at preventing actual disease?' " explains scientist L.J.

"It's possible that the [COVID-19] vaccine will reduce the severity of disease" in the other 50% who do get sick, says physician Bill Miller of The Ohio State University College of Public Health.

While it's true that 50 out of 100 people who get the vaccine could still end up getting infected with the coronavirus, "hopefully, the majority will have milder disease," Ison says.

And in order to get 40% of a population immune through vaccination — if you have a vaccine with 50% efficacy — "you're going to have to vaccinate 80% of the population," says Carlos del Rio, an infectious disease expert at Emory University.

For starters, Michael Ison says, there are logistical challenges to getting everyone vaccinated, given the mammoth undertaking of manufacturing and distributing a vaccine.

What's more, most of the candidate vaccines now being tested would require two separate doses to trigger their most effective protection, Ison says.

It's also still not known how long the immunity gained through any of the current COVID-19 vaccine candidates will last or to what extent the virus might mutate or change in a way that makes that vaccine less effective.

"What we will only know once we have the vaccine [approved and rolled out] is how effective it is, how long lasting the protection is and how much pressure it applies to the virus to change, to escape the pressures of the vaccine," Ison says?

This means half the participants get the real vaccine and "half the patients get a placebo — a water shot," Ison says.

"What we would like to see is that there's a difference in the rate of COVID-19 — with a much lower rate in patients who get the vaccine than in the patients who get the placebo," Ison says?

Ison says it's too soon to know if obesity or age will be a factor in the immune response to the COVID-19 vaccine candidates now being considered for use

In research results so far, he says, "the small, phase one data shows that you can generate antibody responses even in older adults, but it'll be these larger studies that will tell us [whether] we need to use higher doses or more doses of the vaccine in certain cases." Results from phase three trials of safety and effectiveness are expected in the coming months