When the researchers measured levels of SARS-CoV-2 spike protein — the part of the virus that enables it to penetrate and infect host cells and what COVID-19 vaccines use to trigger immune responses against the virus — they found that some individuals with PVS, even those without evidence for infection, had higher levels of spike protein than controls. Typically spike protein can be detected for a few days after vaccination, but some participants with PVS had detectable levels more than 700 days after their last vaccination. Persistent spike protein has been associated with long COVID as well.

For the first time, the FDA is planning to base its decision about whether to authorize new boosters on studies involving mice instead of humans.

"For the FDA to rely on mouse data is just bizarre, in my opinion," says John Moore, an immunologist at Weill Cornell Medicine in New York. "Mouse data are not going to be predictive in any way of what you would see in humans."

Vaccine effectiveness against Omicron

The researchers found a significantly increased risk of developing a symptomatic Omicron case compared to Delta for those who were two or more weeks past their second vaccine dose, and two or more weeks past their booster dose (for AstraZeneca and Pfizer vaccines).

Depending on the estimates used for vaccine effectiveness against symptomatic infection from the Delta variant, this translates into vaccine effectiveness estimates against symptomatic Omicron infection of between 0% and 20% after two doses, and between 55% and 80% after a booster dose. Similar estimates were obtained using genotype data, albeit with greater uncertainty.