by Ivan Couronne Agence France Presse WASHINGTON, United States (AFP) — 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, the other by Moderna and the US National Institutes of Health — are about 95 […]