CDC shortens its recommended isolation period for some people who test positive for Covid-19, FDA warns rapid tests "do detect the omicron variant but may have reduced sensitivity," and more in this week's roundup of Covid-19 news.

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• CDC on Monday released updated isolation and quarantine guidelines for individuals who have tested positive for Covid-19 or have been exposed to the virus. According to the updated recommendations, individuals who test positive for Covid-19 should isolate for five days instead of the previously recommended 10 days. After those five days, CDC said, "[i]f you have no symptoms or your symptoms are resolving … you can leave your house." The agency also shortened the recommended quarantine time following an exposure to someone with Covid-19 to five days, followed by five additional days of masking. However, CDC added that vaccinated individuals who have recently completed their initial vaccine course or received a booster can simply mask for 10 days rather than quarantining. CDC Director Rochelle Walensky said, "With a really large anticipated number of cases [from omicron], we also want to make sure we can keep the critical functions of society open and operating." However, many health care experts expressed concern that the new isolation period may be too short, saying that some infected individuals are still contagious after five days. "I suspect what it will do is result in at least some people emerging from isolation more quickly, and so there'll be more opportunities for transmission and that of course will accelerate the spread of Covid-19," said Yonatan Grad, an associate professor of immunology and infectious diseases at the Harvard T.H. Chan School of Public Health. The New York Times reports that some scientists outside CDC preferred requiring a negative rapid test before allowing an individual to leave isolation—although rapids tests are currently in very short supply in the United States. (Fox, CNN, 12/28; Huang, "Shots," NPR, 12/28; Mueller, New York Times, 12/28)

• CDC on Tuesday estimated that the omicron variant had accounted for around 59% of U.S. Covid-19 cases as of Dec. 25—and it revised its prior estimate for omicron's prevalence as of Dec. 18 from 73% to just 23%. According to CDC, uncertainty surrounding the pace of omicron's spread led to the prior, erroneous prediction. "There was a wide predictive interval posted in last week's chart, in part because of the speed at which omicron was increasing," CDC spokesperson Jasmine Reed said to Politico. "We had more data come in from that timeframe and there was a reduced proportion of omicron." (Romo, NPR, 12/28; Lim, Politico, 12/28; Herman, Axios, 12/28)

• FDA on Tuesday warned that Covid-19 antigen tests—a category of tests that includes over-the-counter rapid testing kits—"do detect the omicron variant but may have reduced sensitivity," according to early data. The agency based its warning on preliminary studies by the National Institutes of Health's (NIH) Rapid Acceleration of Diagnostics (RadX) initiative, which used samples from patients infected with the omicron variant. Epidemiologist Michael Mina—the chief science officer at eMed, and a prominent proponent of rapid testing—suggested on Twitter that the likely issue is that people infected with omicron may be contagious at lower viral loads than people infected with prior variants. For its part, FDA did not specify which antigen tests were used in the study and did not say that people should stop using antigen tests. "The FDA continues to authorize the use of these tests as directed in the authorized labeling and individuals should continue to use them in accordance with the instructions included with the tests," the agency said in a statement. "Antigen tests are generally less sensitive and less likely to pick up very early infections compared to molecular tests." (Lim, Politico, 12/29; Clark, NBC News, 12/28)

• A new study published by South African scientists this week suggests that individuals who have recovered from an omicron infection may have better immune protection against future infections from other Covid-19 strains, including the delta variant. As a result, the researchers said, the omicron variant could eventually dominate and displace the delta variant. In the immediate future, omicron is expected to trigger a surge of cases that will place a massive strain on economies and health care systems globally, the New York Times reports. But if omicron overtakes delta and ultimately proves to be milder than past variants, the scientists' findings suggest, "the incidence of Covid-19 severe disease would be reduced and the infection may shift to become less disruptive to individuals and society." (Zimmer, New York Times, 12/28; Kimball, CNBC, 12/28)

• Roche last week announced that FDA had granted emergency use authorization (EUA) for its Covid-19 At-Home Test. FDA's EUA was granted through the NIH's RADx program, which aims to improve access to high-quality over-the-counter Covid-19 tests. The tests will be available to purchase over-the-counter in pharmacies and retailers nationwide. According to Roche, its tests use "a simple anterior nasal swab sample that can be conveniently self-collected and self-tested by individuals aged 14 years and older, and by an adult for children aged 2-13 years old." They can produce accurate results in roughly 20 minutes for all known Covid-19 variants, including omicron. (Roche media release, 12/24)

• Texas Children's Hospital and Baylor College of Medicine on Tuesday announced that the Drugs Controller General of India (DCGI) had granted EUA approval for Corbevax, a protein sub-unit Covid-19 vaccine. The technology for the vaccine antigen was developed at Texas Children's Hospital's Center for Vaccine Development (CVD). It uses a traditional recombinant protein-based technology that will allow for large-scale production—which may be scaled up rapidly to meet global demands. Corbevax will launch first in India. According to Maria Elena Bottazzi, professor and associate Dean of the National School of Tropical Medicine at Baylor and co-director of the CVD, "Protein-based vaccines have been widely used to prevent many other diseases, have proven safety records, and use economies of scale to achieve low-cost scalability across the world." She added, "Our decade-long studies advancing coronavirus vaccine prototypes has led to the creation of this vaccine, which will fill the access gap created by the more expensive, newer vaccine technologies and that today are still not able to be quickly scaled for global production." (Texas Children's Hospital news release, 12/28)