This week's roundup of some of the latest scientific studies on the coronavirus and efforts to find treatments and vaccines for COVID-19 explores infection risk in early pregnancy, heart failure in patients and a trial that aims to reinfect survivors.

Placental infection more likely in early pregnancy

The coronavirus that causes COVID-19 rarely infects the placenta, but new research suggests that when such an infection does occur it is more likely to happen early in pregnancy.

Analyzing 12 placentas from healthy women, ranging in gestational age from 5 weeks to 36 weeks, researchers found that the cells in the placenta that become infected with the coronavirus have the surface protein ACE2, which the virus uses as a gateway for entry with its spike protein.

Late in pregnancy, the ACE2 proteins are positioned on cells in a way that does not expose them to the virus circulating in the mother's blood, possibly protecting the placenta from infection, said study co-author Dr. Drucilla Roberts of the Massachusetts General Hospital in Boston. This "protective" positioning pattern was less often seen in early gestation placentas when ACE2 was typically present over the entire cell circumference, the study found. That suggests "increased vulnerability of the early placenta to infection," Roberts said.

"As more pregnant women recover from first trimester SARS-CoV-2 infection, it is important to remain vigilant to possible placental infection" and transmission from mother to fetus, the researchers said in a report published in Journal of Infectious Diseases. How often that happens, and effects on the fetus and newborn remain unknown, they added.

New trial to reinfect COVID-19 survivors

British scientists have launched a trial that will deliberately re-expose COVID-19 survivors to the coronavirus to examine their immune responses and see if they become reinfected. The information from the so-called challenge trial "will allow us to design better vaccines and treatments, and also to understand if people are protected after having (COVID-19), and for how long," said study leader Helen McShane of the University of Oxford.

The first stage of the trial will seek to establish the lowest dose of the coronavirus needed in order for it to start replicating in about 50% of the volunteers while producing few to no symptoms. A second phase will infect different volunteers with that standard dose. Everyone will quarantine for at least 17 days, and anyone who develops symptoms will receive a monoclonal antibody treatment manufactured by Regeneron Pharmaceuticals Inc.

Researchers leading a separate U.K. study are administering the virus to volunteers who were not previously infected. "These challenge studies ... will significantly improve our understanding of the dynamics of virus infection and of the immune response, as well as provide valuable information to help with the on-going design of vaccines and the development of anti-viral therapies," Lawrence Young, a virologist at Warwick Medical School in the U.K., said in a statement.

Patients face low risk of new heart failure

Patients hospitalized with COVID-19 may develop heart failure even if they do not have a previous history of heart disease or cardiovascular risk factors, although the risk is low, according to a study published in the Journal of the American College of Cardiology.

Among 6,439 hospitalized COVID-19 patients, doctors found 37 patients (0.6%) with no history of heart failure who developed new cases during their hospitalization – including eight in their 40s, on average, who had no previous cardiovascular disease or risk factors. It is not clear yet whether new-onset heart failure in these patients is an indirect effect of critical illness or a direct effect of the virus invading the heart, said study co-author Dr. Anu Lala of the Icahn School of Medicine at Mount Sinai in New York, in a statement.

"Importantly," she added, "though symptoms of heart failure – particularly shortness of breath – can mimic symptoms associated with COVID-19, being alerted to the findings of this study may prompt clinicians to monitor for signs of congestion more consistent with heart failure than COVID-19 alone."

Patients may not pose highest COVID-19 risk for hospital staff

U.S. health care workers on the frontlines of the pandemic who become sick with COVID-19 are more likely to have acquired the infection in the community than through patient care, new research suggests.

At a major Wisconsin medical center, researchers investigated likely sources of infections by analyzing the gene sequences of the virus obtained on swab samples from 95 health care workers and their patients. Only 11% of participants' infections could be traced to a coworker and only 4% to a patient, the researchers reported in Clinical Infectious Diseases.

They said their observations align with recent studies evaluating health care-associated infections in the Netherlands and in the U.K., and with another recent study that found the most important risk factor for COVID-19 was the rate of the disease in surrounding communities, not workplace factors.

"It appears that health care personnel most commonly become infected with SARS-CoV-2 via community exposure," the researchers conclude. "This emphasizes the ongoing importance of mask-wearing, physical distancing, robust testing programs and rapid distribution of vaccines."