The U.S. Food and Drug Administration (FDA) authorized the use of the updated vaccines at the end of August.

The two newly approved vaccines are known as bivalent boosters because they offer protection against both the original strain of COVID-19 and the Omicron BA.1 subvariant.

They are also expected to be effective against the United States’ dominant strains BA.4 and BA.5, which have a greater ability to evade vaccine-acquired immunity than their predecessors.

One of the approved bivalent vaccines is manufactured by Pfizer and BioNTech, and the other is made by Moderna.

What are the side effects of the new boosters?

Both updated boosters appeared to cause mild side effects in some individuals who participated in human testing.

In a clinical trial involving 800 adults who had already received their primary course of COVID vaccines and a third booster dose, the most commonly reported side effects included:

• Redness and swelling at the injection site
• Fatigue
• Headaches
• Muscle pain
• Joint pain
• Chills
• Swelling of the lymph nodes in the arm where the injection was administered
• Nausea and/or vomiting
• Fever

When it came to the Pfizer-BioNTech bivalent vaccine, the most frequent side effects reported in a clinical trial of 600 over-55s were:

• Redness and swelling at the injection site
• Fatigue
• Headaches
• Muscle pain
• Chills
• Joint pain
• Fever

The side effects are very similar to those reported by people who received the original, so-called monovalent COVID-19 vaccines.

Health officials have stressed that the bivalent vaccines are safe to use.

Anyone aged 12 and older can be given the Pfizer-BioNTech bivalent booster, while Moderna’s shot can only be administered to adults over the age of 18.

To be eligible, individuals must have received their last dose of a COVID vaccine at least two months ago.

While the approval of the new boosters has been heralded “a new phase” in America’s COVID response by President Joe Biden, U.S. officials have faced criticism for approving them before human trials are complete, with critics warning that such a move could increase vaccine hesitancy.

Dr. Anthony Fauci, the White House’s chief medical adviser, said in a press briefing earlier this month that health authorities in the U.S. “fully expect” the updated vaccines to offer better protection than existing shots against the currently circulating strains of the virus.

“Get your updated COVID-19 shot as soon as you are eligible in order to protect yourself, your family, and your community against COVID-19 this fall and winter,” he said.

Pfizer and Moderna’s bivalent boosters have also been approved in the U.K., where they will be administered as a part of the country’s fall vaccination drive.

Pharmacies and other facilities offering the new bivalent vaccines in the U.S. can be found on Vaccines.gov.