Moderna Inc. said on May 5 early human trial data shows that a third dose of either the company’s current Covid-19 shot or an experimental new vaccine candidate increases immunity against variants of Covid-19 first found in Brazil and South Africa.

Total global spending on Covid-19 vaccines is projected to reach $157 billion by 2025, driven by mass vaccination programs underway and “booster shots” expected every two years, according to a report by U.S. health data company IQVIA Holdings Inc. released on April 29.

The United States is preparing for the possibility that a booster shot will be needed between nine to 12 months after people are initially vaccinated against Covid-19, a White House official said on April 15.

China’s CanSino Biologics Inc. said the efficacy rate for the company’s single-dose Covid-19 vaccine may fall over time although it should still have a rate of 50 percent or more five to six months after inoculation.