A year ago, Chinese health experts had hoped the country could safely reopen to the world by now, as it attained herd immunity against the coronavirus.

China achieved last month the herculean goal of vaccinating more than 80 percent of its 1.4 billion people with two doses. But far from reopening, the country has returned to its harshest controls in two years, as it seeks to contain the highly contagious omicron variant.

The renewed lockdowns reflect official concerns about whether China’s vaccines can hold up against omicron, as well as the looming challenge of supplying the population with booster shots before the efficacy of their first two doses diminishes.

The Sinopharm and Sinovac shots in widespread use in China are “inactivated virus” vaccines, a tried-and-true method that has the downside of a higher incidence of breakthrough infections. Chinese scientists say they are working to develop more-effective mRNA vaccines, but it remains unclear whether those can reach market.