Drugmakers Pfizer Inc, BioNTech and Moderna Inc are expected to reap billions of dollars from COVID-19 booster shots in a market that could rival the $6 billion in annual sales for flu vaccines for years to come, analysts and healthcare investors say.

Axar.az reports that for several months, the companies have said they expect that fully inoculated people will need an extra dose of their vaccines to maintain protection over time and to fend off new coronavirus variants.

Now a growing list of governments, including Chile, Germany and Israel, have decided to offer booster doses to older citizens or people with weak immune systems in the face of the fast-spreading Delta variant.

Pfizer, along with its German partner BioNTech , and Moderna have together locked up over $60 billion in sales of the shots just in 2021 and 2022. The agreements include supply of the initial two doses of their vaccines as well as billions of dollars in potential boosters for wealthy nations.

Going forward, analysts have forecast revenue of over $6.6 billion for the Pfizer/BioNTech shot and $7.6 billion for Moderna in 2023, mostly from booster sales. They eventually see the annual market settling at around $5 billion or higher, with additional drugmakers competing for those sales.

The vaccine makers say that evidence of waning antibody levels in vaccinated people after six months, as well as an increasing rate of breakthrough infections in countries hit by the Delta variant, support the need for booster shots.