Older adults who have had Covid-19 are more likely to develop shingles than those who have not, babies born to vaccinated mothers have a lower risk of testing positive for Covid-19, and more in this week's roundup of Covid-19 news.

• People who experience "viral rebound" after taking Pfizer's antiviral Covid-19 medication Paxlovid may be contagious and not know it since they may be asymptomatic, researchers say. "People who experience rebound are at risk of transmitting to other people, even though they're outside what people accept as the usual window for being able to transmit," said Michael Charness, from Veterans Administration Medical Center in Boston. According to Charness, he and his colleagues, as well as researchers from Columbia University, recently identified two patients experiencing viral rebound who transmitted the coronavirus to others. One patient was a 67-year-old man who transmitted the virus to a six-month-old child, while the other patient was a 63-year-old man who transmitted the virus to two other family members. "It indicates that you can transmit during rebound even before you develop symptoms," Charness said. "And you know, we studied a small number of people. It's certainly conceivable that there are other people out there who don't have symptoms and still have a viral rebound." Last week, CDC released new guidance for patients who experience viral rebound after taking Paxlovid, recommending an additional five days of isolation and an additional 10 days of masking. (Goodman, CNN, 5/31)

• Breakthrough infections from the omicron variant may increase immunity more effectively than a second booster dose, according to two new studies. In the first study, which was conducted by BioNTech, researchers found that people who had breakthrough omicron infections had better b-cell responses than people who had received a booster but not been infected. In the second study, which was conducted by the University of Washington in collaboration with Vir Biotechnology, researchers compared blood samples of people who were vaccinated and had breakthrough infections with delta or omicron, those who had been infected first and were then vaccinated, those who were only vaccinated, and those who infected but never vaccinated. Overall, vaccinated people who had breakthrough omicron infections produced antibodies that were protective against other coronavirus variants. In comparison, unvaccinated people who were infected by omicron did not have a similarly strong immune response. However, immunity, both from boosters and infections, wanes after several months. With a significant surge in cases expected in the fall, vaccine manufacturers are currently working to develop next-generation vaccines. Ashish Jha, the White House Covid-19 coordinator, said he believes these new vaccines "are going to provide a much, much higher degree of protection against the virus that we will encounter in the fall and winter." (Lodewick, Fortune, 5/16)

• Infants born to vaccinated mothers are less likely to test positive for the coronavirus, according to a new study published in JAMA Internal Medicine. For the study, researchers examined data from Norwegian birth registries and the Norwegian Immunization Register between September 2021 and February. Women who received either a second or third dose of an mRNA Covid-19 vaccine after 84 gestational days and up to 14 days before delivery were considered vaccinated. In total, 21,643 newborns were included in the study, with 45% born to women vaccinated during their last two trimesters of pregnancy. Overall, 906 infants tested positive for the coronavirus on a PCR test within their first four months of life. During the delta period before Jan. 1, 2022, incidence rates were 1.2 per 10,000 follow-up days for infants of vaccinated mothers and 3.0 for infants of unvaccinated mothers. During the omicron period, the incidence rates increased to 7.0 and 10.9 per follow-up days for vaccinated and unvaccinated mothers, respectively. In addition, the researchers found that among 824 infants whose mothers had a third vaccine dose during pregnancy, none tested positive in the delta period, and the risk of a positive test was lower for those with three doses compared with those with two doses during the omicron period. Compared with infants of unvaccinated mothers, infants whose mothers were vaccinated during their pregnancy had a 33% lower risk of testing positive within four months of being born. "The findings of this study provide early evidence to suggest that infants benefit from passive protection from SARS-CoV-2 infection following maternal COVID-19 vaccination during pregnancy," the researchers wrote. (Walker, MedPage Today, 6/1)

• Individuals ages 50 and older who contracted Covid-19 have an increased risk of developing shingles within six months, according to a study published in Open Forum Infectious Diseases. For the study, researchers analyzed data on roughly 2 million people ages 50 and older from the U.S. MarketScan Commercial Claims and Encounters and Medicare Supplemental and Optum Clinformatics Data Mart databases. Of the participants, 394,677 had been diagnosed with Covid-19, and 1,577,346 had never been infected by the coronavirus. None of the participants had been vaccinated against either shingles or the coronavirus. Overall, individuals who had Covid-19 were 15% more likely to develop shingles within six months than those who had never been infected. In addition, this risk increased to 21% for individuals who had been hospitalized because of severe Covid-19. According to the researchers, Covid-19 may reactivate the varicella-zoster virus, which causes chickenpox in children and shingles in older adults. Currently, around 1 million adults develop shingles every year, and one in three adults will have shingles in their lifetime. To prevent shingles, CDC recommends adults ages 50 and older, as well as those who are immunocompromised, be vaccinated against it. (Searing, Washington Post, 4/19; Bhavsar et al., Open Forum Infectious Diseases, 3/9)

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• Recipients of solid organ transplants who are prescribed Paxlovid for Covid-19 should first adjust certain immunosuppressive drugs to prevent complications, according to a study published in Open Forum Infectious Diseases. In a small case series, two transplant patients who were receiving tacrolimus were admitted to the hospital after testing positive for Covid-19 and being prescribed Paxlovid. At the hospital, the patients received the antibiotic rifampin to reverse tacrolimus toxicity. According to the researchers, solid organ transplant recipients are usually prioritized to receive Paxlovid to prevent severe Covid-19 outcomes, but "drug-drug interactions between ritonavir [part of Paxlovid] and tacrolimus are underappreciated by non-transplant providers" and can lead to "suppressed metabolism and supratherapeutic levels of tacrolimus." Based on these patients' experiences, the researchers recommended holding tacrolimus during Paxlovid's five-day course, checking tacrolimus concentrations on the second and third days, and monitoring patients after they reinitiate the drug. In addition, the researchers called for authorities to take action, saying "a 'Boxed Warning' added directly to medication packaging, provider fact sheets, and drug-interaction software ... would mitigate the dire consequences of these drug interactions" for solid organ transplant recipients. (Walker, MedPage Today, 5/13)