Novavax, based in Maryland, which is developing one of the 12 vaccines that are in the final phase of clinical testing, has enrolled more than 9,000 participants in Britain since beginning a Phase 3 trial there in September. Novavax's president for research and development, Dr. Gregory Glenn, told analysts last week that he was "feeling very good about our ability to collect cases."
Britain reported more than 21,000 new cases on Monday, part of a surge that picked up around the start of October.
Dr. Glenn added: "If you're in the right spot at the right time, you can very quickly accumulate cases. I think we are in the right spot at the right time in the U.K." Novavax plans to start a Phase 3 trial in the United States this month.
Vaccine developers select different parts of the world to conduct their trials for several reasons. They need to ensure that the vaccine has been tested on populations that reflect the world's diversity. They also must make sure they can enroll participants in regions where the virus is spreading.
"What is a hot spot initially will not necessarily stay a hot spot," said Dr. Dean of the University of Florida, who has studied how to make clinical trials flexible enough to work during epidemics.
That's why developers testing Covid-19 vaccines have been carefully watching where the virus is surging to make decisions about where to set up sites to enroll participants.
Johnson & Johnson is testing one version of its vaccine in the United States and other countries. The trials -- which are set up or planned at more than 200 locations globally, including in 30 states -- were chosen in part by modeling the case numbers in different regions, said Jake Sargent, a company spokesman.
Johnson & Johnson announced on Monday that it had started another trial of its vaccine, testing two doses instead of one. The company aims to enroll 30,000 participants in Belgium, Colombia, France, Germany, the Philippines, South Africa, Spain, Britain and the United States. Johnson & Johnson said the locations were chosen "in countries and areas with high incidence of Covid-19."