The shortest known time between coronavirus infections is 20 days, the omicron subvariant BA.2.12.1 is gaining prevalence in the United States, and more in this week's roundup of Covid-19 news.

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• Moderna on Thursday asked FDA to authorize its Covid-19 vaccine for children under 6. Based on clinical trial data, the company's vaccine for children, which consists of two 25-microgram doses, was 51% effective against symptomatic infection for children between 6 months and 2 years and 37% effective for children ages 2 to 6. In addition to its vaccine for children under 6, Moderna has also requested FDA authorize its vaccine for children ages 6 to 11 and adolescents ages 12 to 17, with a spokesperson saying the company would complete its data submission for these groups in the next two weeks. During a Senate oversight heating, Peter Marks, who oversees vaccine regulation for FDA, suggested the agency might consider Moderna's applications for children under 18 as a whole rather than individually. Separately, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said FDA may review children vaccine data from Moderna and Pfizer-BioNTech simultaneously to directly compare the two vaccines and not "confuse people." According to the New York Times, federal regulators are unlikely to review Moderna's application before June, when FDA's outside advisory panel is scheduled to meet on vaccines for young children. (LaFraniere, New York Times, 4/28; Doherty, Axios, 4/28; AP/Modern Healthcare, 4/28; LaFraniere, New York Times, 4/26)

• FDA on Monday expanded its approval for the IV antiviral Veklury, also known as remdesivir, to children under 12, making it the first Covid-19 treatment approved for this age group. According to MedPage Today, Veklury is now approved for use in children as young as 28 days who weigh at least three kilograms. The drug may be used for either hospitalized patients or those at risk of severe outcomes. "As COVID-19 can cause severe illness in children, some of whom do not currently have a vaccination option, there continues to be a need for safe and effective COVID-19 treatment options for this population," said Patrizia Cavazzoni, director of the FDA's Center for Drug Evaluation and Research. "Today’s approval of the first COVID-19 therapeutic for this population demonstrates the agency’s commitment to that need." (Chen, Axios, 4/25; Reuters, 4/25; Walker, MedPage Today, 4/25)

• Booster protection against hospitalization from Covid-19 declines after three months, according to a new Kaiser Permanente study published in The Lancet Respiratory Medicine. For the study, researchers analyzed Kaiser Permanente patient records between December 2021 and Feb. 6, 2022. In total, there were 11,123 hospital admissions or ED visits, and all patients included in the analysis had received at least three doses of the Pfizer-BioNTech vaccine. Overall, the researchers found that a booster dose was 80% to 90% protective against hospital admissions and ED visits caused by both the delta and omicron variants during the first three months of receiving it. However, after three months, protection against hospitalization from omicron decreased to 55%, while protection against ED visits decreased to 53%. "Pfizer BioNTech COVID-19 booster doses significantly improve protection against omicron, although that protection seems to wane after three months against emergency room visits, and even for hospitalization," said Sara Tartof, an epidemiologist in Kaiser Permanente's department of research and evaluation and the study's lead author. "Trends in waning against delta-related outcomes were generally similar to omicron but with higher effectiveness at each time point than those seen for omicron." (Carbajal, Becker's Hospital Review, 4/25)

• The shortest known time between separate coronavirus infections is 20 days, according to a case report presented at the European Congress of Clinical Microbiology & Infectious Diseases (ECCMID). According to researchers, a 31-year-old female health care worker in Spain first tested positive for Covid-19 on a PCR test on Dec. 20, 2021. She was fully vaccinated and had received a booster dose 12 days earlier. After this positive test, the patient, who was asymptomatic, self-isolated for 10 days before returning to work. Then, on Jan. 10, 2022, just 20 days after her original positive test, the patient began feeling unwell and tested positive on another PCR test. According to whole genome sequencing, there had been two different coronavirus variants, with the first being delta in December and the second being omicron in January. "This case highlights the potential of the omicron variant to evade the previous immunity acquired either from a natural infection with other variants or from vaccines," said Gemma Recio, one of the study's authors. "In other words, people who have had COVID-19 cannot assume they are protected against reinfection, even if they have been fully vaccinated." (Gleeson, Becker's Hospital Review, 4/21; ECCMID press release, 4/20)

• Pfizer and BioNTech on Tuesday requested FDA authorize a booster dose of their Covid-19 vaccine for children ages 5 to 11. In an announcement, Pfizer said data from a Phase 2/3 trial indicated children in that group saw a "strong immune response" after receiving a booster dose six months after their primary vaccine series. If authorized, the dose would be the first booster available to children under 12. However, some health experts said a third dose may not be necessary for children at this point. "It may be that over time, those two doses don't protect against serious illness, in which case one could reasonably [receive] a third dose," said Paul Offit, who leads the Vaccine Education Center at Children's Hospital at Philadelphia. "But for right now, protection against serious illness appears to be holding up." In addition, it is not clear how much demand there will be for booster doses in this age group, according to NPR. Currently, CDC data shows that only 28.4% of children ages 5 to 11 are fully vaccinated, and only around 35% have received an initial dose. (Chen, Axios, 4/26; Stein, "Shots," NPR, 4/26; Choi, The Hill, 4/26)

• According to CDC data, the prevalence of the omicron subvariant BA.2 is decreasing in the United States as a new subvariant continues to spread. For the week ending April 23, BA.2 made up 68.1% of all new Covid-19 cases, down from 74.9% the week ending April 9. In comparison, the subvariant BA.2.12.1 made up 28.7% of new Covid-19 cases the week ending April 23, up from 13.7% the week ending April 9. Researchers estimate BA.2.12.1 has a 27% growth advantage over BA.2, which is already more transmissible than the original omicron BA.1 variant. BA.2.12.1, as well as a related subvariant BA.2.12, are currently driving a surge in Covid-19 cases in New York. Based on HHS data, Covid-19 cases nationwide have increased 53% over the past two weeks as of April 25. (Bean, Becker's Hospital Review, 4/26)