“We do not have the information that’s necessary to make the recommendation on whether or not a booster will be needed,” Soumya Swaminathan, the World Health Organization’s chief scientist, said in a Zoom interview Friday. The “science is still evolving.” Such a call is “premature” while high-risk individuals in most of the world haven’t yet completed a first course of vaccination, Swaminathan said. Data from countries introducing precautionary extra inoculations later this year -- particularly for vulnerable people whose immunity to SARS-CoV-2 may wane faster -- will inform WHO’s guidance, she said... Early data from the U.K., Spain and Germany suggest a “mix-and-match” regimen using two different types of vaccines generates more pain, fever and other minor side effects compared with two doses of the same inoculation, the WHO’s Swaminathan said.