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WASHINGTON (AFP) – To find out if Covid-19 experimental vaccines are safe and effective, researchers design clinical trials involving thousands of volunteers, divided into groups that receive either the investigational drug or a placebo.
Data from those trials has shown that two vaccines, one developed by Pfizer and BioNtech, the other by Moderna and the US National Institutes of Health, are approximately 95 percent effective.
Here’s how the tests work:
Who runs the clinical trials?
The US pharmaceutical company Pfizer itself has directed and financed a clinical trial of its vaccine in which almost 44,000 people participated in the United States, Germany, Turkey, South Africa, Brazil and Argentina.
Volunteers who are accepted receive two doses 21 days apart and are regularly monitored.
The Moderna-NIH vaccine is given 28 days apart at locations in the US with about 30,000 volunteers, in a taxpayer-funded trial portion.
The trials are “blind,” meaning that neither the volunteers nor the healthcare providers know who gets the drug and who gets saline as a placebo.
How do we know if the vaccines worked?
The volunteers have not been deliberately infected with the virus. This option, known as a challenge test, was considered but was deemed too risky given that there are no Covid-19 treatments that have been shown to be highly effective.
Instead, volunteers are expected to get on with their lives as normal and given the same advice on wearing masks, distancing themselves and hand washing as everyone else.
Over time, a certain number become infected naturally, and since they are regularly monitored, the trial researchers quickly find out.
If the vaccine is effective, the number of people who get sick in the vaccine group must be less than the number of people who get sick in the placebo group.
The goal is to find a difference large enough to exclude the possibility that it happened by chance. Statistical methods come into play to achieve a predetermined level of certainty.
It’s worth noting that the main goal is to keep people from getting sick, and good vaccines also protect people from serious diseases.
Preventing people from being infected and therefore being able to transmit the virus is a secondary goal for Moderna.
Moncef Slaoui, head of the US government’s Operation Warp Speed, said Wednesday that he hopes the vaccines will also achieve these goals, albeit with a lower efficacy rate.
Why take a placebo?
The only way to know if a vaccine works is to provide 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 age, underlying conditions, and the amount of risky behavior they engage in.
That’s why trials need to be scaled up to tens of thousands of people to eliminate confounding factors.
Who analyzes the data?
To ensure that scientific rigor is maintained, data is protected from companies and analyzed by experts in a “security and data monitoring board” whose membership is kept secret to avoid 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 belonged to which group.
They can then inform the manufacturer in case of conclusive results and the manufacturer can use the data, if 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.
95 percent effective?
Pfizer’s efficacy analysis included 170 people who became ill with Covid-19 in the seven days after the second dose, which might seem like a small number of people compared to the total number of trial participants (44,000).
But suffice it to say that the result is statistically significant with a high degree of certainty.
This is because 162 people who got sick took the placebo and only eight were in the vaccine group – a difference so large that it is extremely unlikely to occur by chance.
An efficacy of 95 percent means that the risk of contracting Covid-19 is reduced by 95 percent in the vaccinated group compared to the placebo group.
Did the placebo group get the vaccine?
All will be transferred to the vaccine arm, but when that will happen is still being decided, Slaoui said. Therefore, the Pfizer and Moderna trials will lose their “control” groups, but the participants will be followed for a further two years to determine how long immunity lasts and whether there are longer-term safety effects.
It will also make it difficult to recruit new people for ongoing vaccine trials that are not yet that close to applying for approval.
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