Coronavirus vaccines tweaked to include the Omicron variant strain can improve protection when used as a booster, the European Medicines Agency and other global health regulators said on Friday.

Following a meeting on Thursday, the EMA said global regulators had agreed on key principles for updating Covid-19 shots to respond to emerging variants.

While the existing coronavirus vaccines continue to provide good protection against hospitalisation and death, the group said, vaccine effectiveness has taken a hit as the virus has evolved.

As such, an Omicron-specific or bivalent booster - meaning a vaccine that includes both the new strain and the original coronavirus strain - could "increase and extend" protection, a statement from the EMA said.

The statement refers specifically to the mRNA vaccines. Both Pfizer Inc and Moderna Inc have been testing retooled versions of their vaccines to include the Omicron variant.

Vaccines which include other variants, for example the Beta variant, might also be considered for use as boosters if clinical trial data demonstrate an adequate level of neutralisation against Omicron and other variants of concern, the statement said.

It follows guidance from the World Health Organization that Omicron-specific boosters could restore protection against emerging strains of the coronavirus.

But it stops short of the position of the regulator in the United States, the Food and Drug Administration (FDA), which said on Thursday that it would seek the inclusion specifically of the newer BA.4 and BA.5 strains of Omicron, currently driving a surge in new infections globally, in any new shots for use domestically.

On Tuesday, the head of a WHO advisory committee that has considered the modified shots said the group preferred BA.1-based boosters, arguing that the variant is more distinct and could generate a broader response than the more recently circulating subvariants.

Top US FDA official Peter Marks said in an interview that regulators from other countries were seriously considering using new boosters based on the BA.1 Omicron variant that caused the massive surge in cases last winter, because those shots can be available sooner than the BA.4/5 based booster the United States plans to use.

The EMA said it would provide more details in coming days.

- Reuters