An Israeli hospital began giving a fourth COVID-19 vaccine dose to a small group of health care workers as part of an effort to study the safety and effectiveness of a second booster shot amid a surge in cases due to the highly transmissible omicron variant. In what is being billed as the first study of its kind in the world, 150 health care workers who received a third dose of the Pfizer-BioNTech vaccine at least four months ago are getting a new shot. Results of the trial will be turned over to Israel’s Health Ministry in around two weeks.

Many officials around the world are likely to keep close tabs on the study considering how Israel was the first to fully launch a vaccine booster program after it realized that immunity decreased over time. Even though there are hints that the omicron variant may lead to milder infections, an increase in infections around the world is already causing a surge in hospitalizations.

The study is starting a week after a panel of medical experts recommended to the Israeli government that medical workers, people over 60, and those with compromised immune systems receive a second booster. Some have warned the recommendation was too premature considering there is no real data on a second booster shot, but officials have said the country couldn’t afford to wait considering the rising cases. “We can’t close our eyes and think happy thoughts,” Health Minister Nitzan Horowitz said. “We have to prepare.”

Nothing has been decided yet, but reports in Israeli media suggest the government may at first limit the second booster to those who are older than 70. And while they make the decision, officials say they also want to get a better handle on what kind of risk the omicron variant represents. “The biggest question is, how significant is omicron? It’s clear to all that it is very contagious. But whether it causes very severe illness—that’s the most significant question,” Gili Regev-Yochay, who is running the trial, said.