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.

Severe Covid-19 may trick the immune system into producing so-called autoantibodies that have the potential to eventually attack healthy tissue and cause inflammatory diseases, researchers warned in a paper published in Nature Communications. Another study suggests that the virus that causes Covid-19 may be getting better at traveling into the air

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.

Eli Lilly said on Sept. 15 the U.S. government bought 388,000 additional doses of the company’s Covid-19 antibody therapy and Regeneron Pharmaceuticals said on Sept. 14 the U.S. government was buying 1.4 million additional doses of the Covid-19 antibody cocktail REGEN-COV., as infections surge due to the fast-spreading Delta variant.

Three U.S. studies suggest Covid-19 vaccines offer strong protection against hospitalization and death, even in the face of the highly transmissible Delta variant, but vaccine protection appears to be waning among older populations, especially among those 75 and older.

What is the key to a robust immunity against SARS-CoV-2? According to a study recently published in The New England Journal of Medicine, it could be a previous SARS infection—more specifically, the one that spread across 29 countries between 2002 and 2004—plus vaccination.