The pandemic has seen mRNA vaccines— Moderna Inc. makes another—being used in scale for the first time. Data from clinical trials so far suggest they are better at stopping Covid, tackling variants, and preventing the virus from spreading in the community. Sinopharm’s and AstraZeneca’s vaccines rely on more traditional methods, and their shots have been shown to have lower efficacy in studies. What’s also apparent, so far, is that though there’s been a surge in cases in Seychelles, very few people are getting seriously ill. “We have only a few people needing intensive care. Two out of 40” in hospitals, said Seychelles President Wavel Ramkalawan in a May 10 interview. “The vaccine will protect people from getting serious symptoms.”... The situation in Seychelles also offers a fresh reminder that our understanding of herd immunity—the theoretical threshold at which the virus can’t find enough hosts to keep spreading—continues to evolve. Scientists once estimated that 55% to 82% of the population would need to have immunity against SARS-CoV-2, either from recovering from an infection or through vaccination. But 17 months into the pandemic, there’s recognition that the threshold may vary depending on the susceptibility of a population to the circulating strains, adherence to physical distancing, mask-wearing, and other practices known to reduce transmission, as well as the season—epidemics typically worsen as either colder or hotter weather encourages people to congregate indoors.