US regulators have approved booster shots for both the Moderna and Johnson & Johnson Covid-19 vaccines.
Those, along with Pfizer, are the three approved boosters in the country.
The Food and Drug Administration has also announced it's allowing people to receive a booster dose of a vaccine different to that of their first or second jab - which is being referred to as mixing and matching vaccines.
Worldwatch's Max Towle speaks to correspondent Ira Spitzer.
