Most breakthrough infections that led to hospitalizations occurred in seniors, Pfizer's antiviral pill may be effective against omicron, and more in this week's roundup of Covid-19 news.

• A preliminary study made public on Wednesday suggests that Moderna's Covid-19 vaccine—much like Pfizer-BioNTech's—is less effective against omicron than the original coronavirus. For the study, researchers analyzed blood samples from 30 people who had received two doses of Moderna's vaccine and found that there was a significant reduction in the effectiveness of neutralizing antibodies against omicron compared to the original coronavirus. "The antibodies that people make after they get the standard two inoculations of the Moderna mRNA vaccine are 50 times less effective against omicron than they are against the original form of the virus," said David Montefiori, a virologist at Duke University who helped conduct the study. However, researchers also looked at blood samples from an additional 17 people who had received a Moderna booster and found that antibody effectiveness against omicron increased to what it would be against delta. "What these results are telling us is that if omicron becomes a dominant variant, it's going to become even more important that people get their boost," Montefiori said. (Stein, "Shots," NPR, 12/15)

• CDC's Advisory Committee on Immunization Practices (ACIP) met on Thursday to review data on the potential risks of a rare but serious blood clotting disorder linked to Johnson & Johnson's (J&J) Covid-19 vaccine. The disorder, which is called thrombosis with thrombocytopenia syndrome, can prevent clotting and cause internal bleeding. According to new FDA data, both men and women may be at elevated risk for the condition after receiving J&J's vaccine, but the rate was highest among women ages 30-49, at 1 in 100,000 people. So far, nine deaths have occurred from the disorder. Based on this data, FDA on Tuesday updated fact sheets for J&J's vaccine to say it shouldn't be used in people who experienced clotting problems after a first dose. However, the agency added that it "continues to find" the vaccine's benefits outweighs its risks. After reviewing the presented evidence, ACIP voted 15-0 to recommend Pfizer-BioNTech's and Moderna's Covid-19 vaccines over J&J's. In a statement, CDC said the J&J vaccine will remain on the market and be available for individuals who are "unable or unwilling" to receive mRNA Covid-19 vaccines. Currently, around 16 million people in the United States have received the J&J vaccine as their primary immunization against Covid-19. (Weiland/Robbins, New York Times, 12/16; Sun/McGinley, Washington Post, 12/15; Walker, MedPage Today, 12/16; Axios, 12/16; Robbins/Jewett, New York Times, 12/17; AP/Modern Healthcare, 12/17)

• Pfizer on Tuesday released new data showing that its antiviral Covid-19 pill, Paxlovid, reduced the risk of hospitalization or death by 88% in high-risk adults when given within five days of symptom onset. This data, which was based on 2,200 unvaccinated participants, confirms the results of a smaller analysis of the drug, the results of which were released last month, the New York Times reports. In addition, the company said that laboratory experiments suggest that the drug can attack a key protein in the omicron variant. "This is quite amazing and potentially transformative," said Sara Cherry, a virologist at the University of Pennsylvania Perelman School of Medicine who was not involved in the study. "If we could keep people out of hospitals, that would have a huge impact on health care." Following Pfizer's report, the Biden administration announced that it had ordered enough of the drug to cover 10 million Americans. Currently, FDA is reviewing Pfizer's data on Paxlovid and is expected to authorize it for emergency use this month, the Times reports. (Zimmer/Robbins, New York Times, 12/14; Reed, Axios, 12/14; Falconer, Axios, 12/15)

• Most breakthrough infections of Covid-19 that resulted in hospitalization were in seniors, according to a Kaiser Family Foundation (KFF) analysis released Wednesday. For the analysis, researchers looked at more than 120,000 hospitalizations involving a Covid-19 diagnosis between June and September. They found that almost 70% of fully vaccinated patients hospitalized with Covid-19 were ages 65 and older, which the researchers called a "disproportionately large share" of breakthrough hospitalizations. However, researchers found that unvaccinated people were more affected by Covid-19 hospitalizations than vaccinated people. Overall, 85% of all Covid-19 hospitalizations between June and September were among the unvaccinated. In addition, unvaccinated patients had a longer median hospital stay (6.7 days) compared to fully vaccinated patients (5.6 days). Unvaccinated patients also had more Covid-related respiratory complications or treatments than those who were vaccinated, which suggests that vaccinated patients may have been hospitalized for non-Covid reasons. (Coleman, The Hill, 12/15)

• Nursing home residents who have received Covid-19 booster doses are 10 times less likely to be infected by the coronavirus compared with residents who have received only two vaccine doses, according to new CDC data. CDC Director Rochelle Walensky on Wednesday presented data showing Covid-19 case rates increasing over the past two months among residents of long-term care facilities who are either unvaccinated or have only received a primary vaccine dosage. In comparison, the rate among residents who have received boosters has remained "stable and low," Walensky said. Based on the data, Walensky encouraged nursing home staff and residents to get Covid-19 booster shots. “Taken together [this] data emphasize[s] the critical importance of boosters to optimize the protection of vaccines over time, and that when boosters are used that protection works,” she said. (Coleman, The Hill, 12/15)

• America's Covid-19 vaccine program helped prevent more than 10.3 million hospitalizations and around 1.1 million deaths by November, according to a new report from the Commonwealth Fund. To evaluate the impact of vaccination, researchers created a model simulating the trajectory of the pandemic without Covid-19 vaccines. Characteristics of three variants (alpha, iota, delta) were included in the analysis model. According to the model, the United States would have experienced 4.9 times more hospitalizations and 3.2 times more deaths from Covid-19 without vaccines. In addition, daily Covid-19 deaths could have potentially spiked to as many as 21,000 per day—almost 5.2 times higher than the peak of 4,000 deaths a day seen in January. (Adams, Becker's Hospital Review, 12/14; Schneider et al., Commonwealth Fund, 12/14)