A study finds relatively few high-risk patients experience "viral rebound" after taking Paxlovid, FDA's potential authorization of Novavax's Covid-19 vaccine is delayed until the agency can fully review the company's manufacturing process, and more in this week's roundup of Covid-19 news.

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• FDA's Vaccines and Related Biological Products Advisory Committee (VRBPAC) on Wednesday voted unanimously to recommend authorizing both Pfizer-BioNTech's and Moderna's vaccines for young children. In a 21-0 vote, VRBPAC voted in favor of Moderna's two-dose series for children ages six months to five years old and Pfizer-BioNTech's three-dose series for children ages six months to four years old. "This is a huge milestone in the pandemic, not because it changes the trajectory of the virus but because of the way we as a country can think about where we are," said Leana Wen, an emergency physician and a professor at George Washington University. "Right now, there are so many parents who have not themselves been able to resume their pre-pandemic lives. There are many families who have put off gatherings or travel or other aspects of normal life. This allows millions of families to change the way they perceive where they are." According to the New York Times, FDA may authorize the two vaccines as soon as Friday, and CDC's advisory committee is meeting on Friday and Saturday to make its own recommendations for the vaccines. If both FDA and CDC authorize the vaccines, shots could be available for children as soon as June 21. In anticipation of federal authorization, the White House has made millions of vaccine doses available for states and health care providers. As of last week, roughly 1.45 million doses of the Pfizer-BioNTech vaccine and around 850,000 doses of the Moderna vaccine have been ordered out of an initial 5 million available doses. (LaFraniere/Weiland, New York Times, 6/15; Reed, Axios, 6/16; Reed, Axios, 6/15; Cohen, Roll Call, 6/15; Herper/Branswell, STAT News, 6/15)

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• Individuals who have only had two Covid-19 vaccine doses have essentially no protection against infection from the omicron variant, according to a new study published in the New England Journal of Medicine. For the study, researchers from Qatar examined data from the country's national databases on Covid-19 vaccination, laboratory testing, hospitalization, and death from Dec. 23, 2021, and Feb. 21, 2022. During this time, the omicron subvariants BA.1 and BA.2 drove a surge of cases in the country. "Overall, the researchers found that individuals who had three vaccine doses as well as prior infection had the strongest protection against infection (77%), followed by those who had two vaccine doses as well as prior infection (55%), those who had three vaccine doses (52%), and those who had a prior infection alone (46%)." However, individuals who received only two vaccines had essentially no protection against infection—although their protection against severe illness remained robust. According to the Los Angeles Times, Covid-19 vaccines have become less effective against infection over time due to the coronavirus evolving and accumulating mutations. "The immune evasion is so much higher" with omicron, making it "essentially a new virus," said Laith Jamal Abu-Raddad, an infectious disease epidemiologist at Weill Cornell Medicine-Qatar and one of the study's authors. However, Abu-Raddad noted that even with protection against infection declining, "[t]he immunity against severe COVID-19 was really very much preserved" for vaccinated patients, regardless of the number of doses they received. (Purtill, Los Angeles Times, 6/15; Altarawneh et al., New England Journal of Medicine, 6/15)

• Relatively few high-risk Covid-19 patients experience "viral rebound" after being treated with Paxlovid, according to a new retrospective study published in Clinical Infectious Diseases. For the study, researchers from Mayo Clinic examined outcomes from 483 high-risk patients who were treated with a five-day regimen of Paxlovid. The median age was 63 years, and 56% were women. Of the patients, only four (<1%) had their Covid-19 symptoms return after treatment. Overall, the symptoms, which included cough, dyspnea, and fatigue, were generally mild and ultimately resolved without additional treatment. Although the number of patients with rebound symptoms was small in this particular study, the authors said the issue of viral rebound after Paxlovid treatment would benefit from further study. "Identifying risk factors may help distinguish patients who are more likely to experience rebound phenomenon," the researchers wrote. "We are unable to define risk factors in this study due to a small number of cases, but it is notable that the four patients with rebound had multiple underlying medical comorbidities and had received SARS-CoV-2 vaccine more than 90 days prior to nirmatrelvir/ritonavir [Paxlovid] therapy." (Reed, Axios, 6/14; Short, MedPage Today, 6/14)

• Covid-19 may be the potential cause of a series of unexplained pediatric hepatitis cases, a new study from Israel suggests—however experts caution that the study is too small to be conclusive. According to the World Health Organization (WHO), more than 700 children in 34 countries have reported probable cases of severe hepatitis without a known cause, and an additional 112 cases are under investigation. Of the confirmed cases, at least 38 children have required a liver transplant, and 10 have died. For the Israel­­ study, researchers examined five patients ages three months to 13 years who were hospitalized at Schneider Children's Medical Center last year for liver injuries. All five patients had tested positive for the coronavirus and were asymptomatic or had mild symptoms before the onset of their liver issues. According to the researchers, the most likely causes of acute hepatitis in these patients was either an abnormal immune response to the coronavirus or multisystem inflammatory syndrome, a post-infection immune reaction that has been known to affect children. "The latest Israeli study adds just a little gasoline to the fire to try to understand the causes of the mysterious hepatitis in children," said Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco. However, he noted that the study is too small to determine a definitive cause. Previously, WHO posited that adenovirus 41, a common cold virus, may be behind the pediatric hepatitis cases, but investigations are still ongoing. (Salzman, ABC News, 6/14; Joffre, Jerusalem Post, 6/14)

• FDA's vaccine advisory committee last week voted in favor of authorizing Novavax's protein-based Covid-19 vaccine, but FDA's final decision on the vaccine will be delayed until the agency can fully review new changes Novavax's manufacturing process. "FDA will carefully review this and any additional information submitted by the firm as part of its ongoing assessment and prior to authorizing the vaccine for emergency use," said agency spokesperson Abby Capobianco. So far, Novavax's vaccine has been authorized in more than 40 countries, including Australia, Canada, and the European Union. However, Peter Marks, head of FDA's vaccine division, said the United States does not base its regulatory decisions on whether other countries have authorized a specific vaccine or treatment. "We take manufacturing very seriously," Marks said. "We don't benchmark ourselves against other countries when it comes to manufacturing, we consider that we have a very high standard, and it's why we're often considered a gold standard for our manufacturing, and particularly in the area of vaccines." (Kimball, CNBC, 6/9)