How to interpret the 'efficacy' rates of coronavirus vaccines

Felix Dlangamandla/Beeld/Gallo Images via Getty ImagesA volunteer receives an injection for a potential vaccine against COVID-19 at the Baragwanath Hospital on June 28, 2020 in Soweto, South Africa. The vaccine, developed by Oxford University’s Jenner Institute, will inoculate 2,000 South Africans.
  • Vaccine efficacy measures how well a vaccine works at preventing disease among vaccinated people, when compared to unvaccinated individuals.
  • The vaccine efficacy numbers for coronavirus injections being reported from Pfizer, Moderna, and AstraZeneca are all preliminary, and based on their trials in tens of thousands of volunteers around the world.
  • Very few vaccines developed are 100% effective at preventing diseases. But, if enough people get their shots, they can prevent the spread of disease near-perfectly anyway through herd immunity.
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95%. 94.5%. 70%. Lots of numbers are being thrown around lately, touting how effective different coronavirus vaccines are looking so far, based on trials on tens of thousands of volunteers around the world.

But what do these percentages really mean?

Vaccine efficacy measures how well a vaccine works at preventing disease in a well-managed clinical trial. The same concept, when applied to how well a vaccine works in the general public, is called vaccine effectiveness.

Either way, this “VE” metric measures how well a vaccine works at preventing disease. So, the closer the efficacy or effectiveness % is to 100, the better a shot works at keeping vaccinated individuals healthy.

Polio vaccines, for example, at 99%-plus, are near-perfect. Annual flu shots, which typically range from 40-60% effective, are not as great at completely halting that disease — but they may make a case of the flu, or another disease like the coronavirus, milder if a person who got the flu shot later gets sick.

How well each coronavirus vaccine is performing so far

Coronavirus vaccine trial UKKirsty O’Connor/PA Images via Getty Images

So far, vaccines for the coronavirus appear to be performing very well at preventing future illnesses, in the trials that vaccine makers have been conducting around the world in recent months.

Pfizer’s shot course is 95% effective at preventing infections, the company reported, after enrolling more than 43,000 volunteers to try out its new mRNA vaccine. A preliminary look at a subset of thousands of those participants suggests the 2-shot regimen does very well at preventing disease: only 8 confirmed cases of the coronavirus have been diagnosed so far in Pfizer’s vaccinated group, whereas 162 coronavirus cases were found in the control group, the company said in a release.

Moderna’s 2-shot course displayed a similar 94.5% efficacy, the company reported last week, with more than 30,000 people enrolled in US-based trials. An initial analysis of 95 coronavirus cases in Moderna’s trial participants revealed 90 cases among control individuals who were “vaccinated” with a fake a placebo shot, compared with just 5 cases among those who received the real treatment, the company said.

On Monday, AstraZeneca chimed in to the vaccine results race too, touting 70% efficacy of its two shot course in trials. Interestingly, a select group of participants who received only a half-dose injection on their first shot, and then the full second shot one month later, seemed to fare better than others in that study: the vaccine appeared 90% effective in the half-dose first group, while among the participants who got both full doses, vaccine efficacy was a lower 62%.

The vaccines will only be truly effective if they reach most people, and that could could take the better part of 2021

Schott glass vialsSchottGlass vials made by Schott

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, recently told the New York Times that in order to achieve herd immunity against the coronavirus, thereby protecting populations from a major disease outbreak, vaccines like Pfizer’s or Moderna’s would need to reach upwards of 70% of people in a population.

Bill Gates agreed, telling Trevor Noah that “we don’t need everyone to take the vaccine.”

“With this level of efficacy, if we could get to 75% dose, then you’ll block the spread of the disease,” Gates said.

In the US, vaccine distribution then will likely take the better part of next year, according to estimates from Morgan Stanley and others. But Fauci cautioned that efficacy is only one part of that equation. A highly effective vaccine, if it is not widely used, doesn’t work well.

“What I would like to see is the overwhelming majority of people get vaccinated so we can essentially really crush this outbreak,” Fauci told the Times last week. “As we get into the fall [2021] we could be quite close to some degree of normality, certainly from the standpoint of the economy: of getting businesses open, of getting sports events being attended to, that is feasible.”

There are still some unanswered questions about exactly how well these new vaccines work, which we likely won’t have answers for until they are distributed widely, to many hundreds of thousands of people.

It’s possible that coronavirus vaccines may not provide people with full-on sterilizing immunity, meaning that vaccinated people could, possibly, still get the virus to replicate in their bodies, and pass it along to others, asymptomatically, even after vaccination, while appearing and feeling perfectly healthy.

But, even if that were to be the case, having such highly-effective vaccines widely available will greatly reduce the burden of death and disease wrought by this virus so far, and improve pandemic conditions around the world next year.

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