To find out whether experimental COVID-19 vaccines are safe and effective, researchers design clinical trials involving thousands of volunteers, divided into groups that receive either the drug under investigation or a placebo.
Data from such trials have shown that two vaccines — one developed by Pfizer and BioNtech, and the other by Moderna and the US National Institutes of Health (NIH) — are about 95 percent effective.
Here is a look at how trials work:
US pharmaceutical Pfizer is leading and financing a clinical trial of its vaccine involving almost 44,000 people in the Argentina, Brazil, Germany, South Africa, Turkey and the US.
Volunteers who are accepted receive two doses 21 days apart and get regular follow-up.
The Moderna-NIH vaccine is given 28-days apart to about 30,000 volunteers at locations across the US in a trial partially funded by US taxpayers.
The trials are “blind” meaning that neither the volunteers nor the healthcare providers know who gets the medicine and who gets a saline solution placebo.
Volunteers have not been deliberately infected with the virus. Infecting volunteers, known as a challenge trial, was considered, but deemed too risky given that there are not any COVID-19 treatments that have proven to be highly effective.
Instead, volunteers are expected to go about their lives as normal, and are given the same advice about mask use, distancing and hand washing as everyone else.
Over the course of time, a certain number naturally become infected, and since they are getting regular follow-up, the trial investigators quickly find out.
If the vaccine is effective, the number of people who fall sick in the vaccine group should be lower than the number of people who get ill in the placebo group.
The aim is to find a difference that is great enough to exclude the possibility that infection occurred by chance. Statistical methods come into play to achieve a predetermined level of certainty.
The primary goal is to prevent people from getting sick, with good vaccines also protecting people from severe illness.
Stopping people from getting infected at all, and thus being able to transmit the virus forward, is a secondary goal for Moderna.
On Wednesday, Moncef Slaoui, chief of the US government’s Operation Warp Speed, said that he expects that the vaccines would achieve these goals, too, although at a lower efficacy rate.
The only way of knowing whether a vaccine works is by providing a “control,” which is a demonstration of what would happen in its absence. Of course, different people have different risk profiles in terms of their ages, underlying conditions and how much risky behavior they engage in. That is why the trials need to be scaled to tens of thousands of people to eliminate confounding factors.
To ensure that scientific rigor is upheld, the data is firewalled from the companies and analyzed by experts in a “data and safety monitoring board,” whose membership is kept secret to prevent political pressure.
Over the course of a trial, these committees will lift the hood on the data that has been collected so far and see which patients belong to which group.
They can then inform the manufacturer in case of conclusive results and the manufacturer can then use the data, if it is favorable, to apply for a marketing authorization.
The boards also closely monitor safety, looking at the frequency and severity of side effects, a key factor in determining regulatory approval.
Pfizer’s efficacy analysis featured 170 people who fell sick with COVID-19 in the seven days following the second dose, which might seem like a small number of people compared with the total number of trial participants (44,000).
However, the number is enough to say that the result is statistically significant with a high-degree of certainty. That is because 162 people who fell ill were on the placebo, but only eight were in the vaccine group — a difference so great that it is extremely improbable to come about by chance.
An efficacy of 95 percent signifies that the risk of catching COVID-19 is reduced by 95 percent in the vaccinated group, compared with the placebo group.
The members of the placebo group are to all be transferred to the vaccine group — but it is still being decided when that will happen, Slaoui said.
The Pfizer and Moderna trials are therefore to lose their “control” groups, but the participants would still be followed for two more years to determine how long the immunity lasts and whether there are longer-term side effects.
Immunity would make it harder to recruit new people to ongoing trials for vaccines that are not yet as close to applying for approval.
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