"This safe and effective vaccine will broaden immunity and potentially improve protections against some variants as we learn to live with this virus."
Such an approach, combining protection against several strains of the same disease is used with flu shots, which are adjusted each year depending on the variants that are circulating and can protect against four influenza strains.
Stephane Bancel, Moderna's chief executive, said it was the first regulatory authorisation for a vaccine aiming to fight the Omicron variant, predicting the booster would have an "important role" to play in protecting people against Covid-19 in the winter.
On Friday, Germany's health minister said the European Medicines Agency might clear tweaked Covid-19 boosters next month.
In June, the US Food and Drug Administration told vaccine makers that any booster shots tweaked for autumn would have to include protection against the newest Omicron variants, meaning BA.4 and BA.5, not the BA.1 subvariant included in Moderna's latest shot.
Last month, the FDA said it was no longer considering authorising a second Covid-19 booster for all adults but would instead focus on revamped vaccines for the autumn that target the newest viral subvariants.
Both Moderna and Pfizer are currently brewing updated versions of their vaccine to include BA.5 in addition to the original Covid-19 virus.
According to the World Health Organisation, the latest global surge of Covid-19 has been driven by omicron subvariant BA.5, which is responsible about 70 per cent of the virus samples shared with the world's largest public virus database. The subvariant BA.5 is even more infectious than the original version of Omicron and has some genetic differences that earlier vaccines might not address.
Scientists have warned that the continued genetic evolution of Covid-19 means drugmakers will likely be one step behind the virus in their efforts to tailor their vaccines.
"The virus is unlikely to stand still and Omicron-targeted immunity, might push the virus down other evolutionary paths," warned Jonathan Ball, a professor of virology at Britain's University of Nottingham. Still, he said the new Moderna vaccine would likely still be protective against severe disease.
Other experts said it was still unknown how effective the new combination vaccine would be.
Beate Kampmann, director of the Vaccine Centre at London's School of Hygiene and Tropical Medicine, said the shot would most likely only offer "partial protection" against the latest Omicron variants including BA.5 since it was developed based on earlier versions of Covid-19.
"How much difference such [combination] vaccines can actually make remains to be seen," she said, noting that the shots have not yet been widely tested in different populations.
