SAO PAULO—Brazil’s death toll from COVID-19 surpassed 500,000 on Saturday as experts warn that the world’s second-deadliest outbreak may worsen. Only 11 percent of Brazilians have been fully vaccinated after manufacturing delays and epidemiologists believe that, with winter arriving in the southern hemisphere and new variants of the coronavirus circulating, deaths will continue to mount even if immunizations gain steam. Brazil has relied overwhelming on a vaccine developed by China’s Sinovac Biotech, which Singapore’s director of medical services at the Ministry of Health Kenneth Mak said on June 18 presented a significant risk of “vaccine breakthrough.” Mak pointed to international evidence reporting that many who have taken the China-made vaccine later became infected with COVID-19. “It’s not a problem associated with Pfizer. This is actually a problem associated with the Sinovac vaccine, and in other countries, they are now starting to think about booster vaccinations, even six months out from an original vaccination …