U.S. regulators are authorizing a fourth dose of either Pfizer or Moderna COVID-19 vaccine for Americans 50 and older, amid concerns over whether the highly contagious Omicron subvariant, BA.2, will trigger another wave south of the border as it has in Europe and China.

The Food and Drug Administration is also approving a fourth dose or second booster for those 12 and older with compromised immune systems.

They need to be given at least four months after the third dose, and are intended to offer more protection against severe disease and hospitalization.

In Canada, fourth doses are already being offered to those in certain high-risk groups such as residents of nursing homes or those who are immuno-compromised.