The Omicron coronavirus variant detected in southern Africa could be the most likely candidate to displace the highly contagious Delta variant, the director of South Africa’s communicable disease institute said on November 30.

The UK Health Security Agency designated a Delta coronavirus subvariant called AY.4.2 as a “Variant Under Investigation,” saying there was some evidence that it could be more transmissible than Delta.

British health authorities, as well as global experts, are closely watching a subtype of the Delta variant that appears to be rising in the UK.

A booster dose of the Covid-19 vaccine developed by Pfizer Inc. and German partner BioNTech SE restored efficacy to 95.6 percent against the virus, including the Delta variant, data released by the companies from a large study showed on Oct. 21.

The U.S. Food and Drug Administration on Oct. 20 authorized booster doses of the Covid-19 vaccines from Moderna Inc. and Johnson & Johnson, and the regulatory agency said Americans can choose a different shot from their original inoculation as a booster.

The Delta variant of the coronavirus does not appear to cause more severe disease in children than earlier forms of the virus, a UK study suggests. Another study found that in Covid-19 survivors, important components of the body’s immune response called memory B cells continue to evolve and get stronger for at least several months, producing highly potent antibodies that can neutralize new variants of the virus.

Worldwide deaths related to Covid-19 surpassed 5 million on Oct. 1, according to a Reuters tally, with unvaccinated people particularly exposed to the virulent Delta strain.

The United States surpassed 700,000 coronavirus-related deaths on Oct. 1, according to a Reuters tally, as officials roll out booster doses of vaccines to protect the elderly and people working in high-risk professions.

Laboratory studies show that Merck & Co.’s experimental oral Covid-19 antiviral drug molnupiravir is likely to be effective against known variants of the coronavirus, including the dominant, highly transmissible Delta, the company said on Sept. 29.

A potential vaccine from China’s Clover Biopharmaceuticals was 67 percent effective against Covid-19 of any degree of severity and 79 percent against the highly infectious Delta variant in a large trial, the company said on Sept. 22.