Covid-19 vaccine developers are making ever bolder assertions that the world will need yearly booster shots, or new vaccines to tackle concerning coronavirus variants, but some scientists question when, or whether, such shots will be needed.

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.