The following is a roundup of some of the latest scientific studies on the novel coronavirus and efforts to find treatments and vaccines for Covid-19, the illness caused by the virus.
Breakthrough infections rare, but potentially contagious
As of April 30, when roughly 101 million Americans had been fully vaccinated against Covid-19, "breakthrough" infections had been reported in 0.01% of them, the US Centers for Disease Control and Prevention (CDC) reported on Tuesday.
Roughly 27% of breakthrough infections were asymptomatic, while in 2% of cases, patients died. The CDC had genetic data for virus samples from 555 breakthrough infections.
Mutated variants of the coronavirus, including those first seen in the UK and South Africa, accounted for 64% of the breakthroughs. In a separate study posted Tuesday on medRxiv ahead of peer review, researchers reported that among 20 fully-vaccinated healthcare workers with breakthrough Covid-19 cases, all were infected with variants.
An earlier study had linked breakthrough infections with low viral loads, suggesting low transmission risks, but "we found many samples in our breakthrough cohort with high viral load," said coauthor Pavitra Roychoudhury of the University of Washington.
"Our work suggests that not all breakthrough infections are at low risk of initiating transmission and, if they did, these infections could lead to the continued spread of variants of concern, particularly in areas with low vaccination rates."
Study refutes anti-vaxxers' pregnancy, breast milk claims
Unfounded claims by anti-vaccine activists that Covid-19 shots from Pfizer/BioNTech, and Moderna will damage the placenta and contaminate breast milk have been refuted by new data.
The vaccines deliver synthetic messenger RNA (mRNA), which instructs the body to make proteins that in turn induce antibodies to attack the coronavirus. Anti-vaxxers claim, with no evidence, that mRNA also induces antibodies that attack a protein called syncytin-1, which is important for the developing placenta during pregnancy.
They also claim mRNA from the vaccines ends up in breast milk. When researchers studied blood samples from 15 women who received at least one dose of the Pfizer/BioNTech vaccine - including two pregnant women and five who were breastfeeding - they saw coronavirus antibodies but no antibodies against syncytin-1.
None of the breastfeeding women had vaccine mRNA in their milk, according to a report posted Tuesday on medRxiv ahead of peer review.
"This small study tells us that it is unlikely that Covid-19 mRNA vaccination will cause complications in pregnancy or fertility through cross-reacting antibodies against syncytin-1, or for breastfed infants through breast milk," the authors said.
Vaccines appear safe for "long Covid" survivors
Covid-19 survivors with lingering symptoms can safely be vaccinated against the coronavirus, a small study suggests. Researchers tracked 36 individuals with "long Covid" who had been hospitalized while acutely ill and who later received at least one dose of either the Pfizer/BioNTech or AstraZeneca vaccine.
Eight months after admission to the hospital, and before vaccination, participants had at least one lingering symptom, and half had at least four symptoms.
Before vaccination, their quality of life was "markedly reduced" from normal, the researchers reported on Monday in Annals of Internal Medicine. One month after vaccination, 71% of their symptoms remained unchanged, 23% of their symptoms were improved, and 6% of symptoms had worsened.
There was no significant worsening in quality-of-life or mental well-being, and outcomes were similar with both vaccines, researchers reported.
The results may reassure people with persistent Covid-19 symptoms that the different types of vaccines developed by Pfizer/BioNTech or AstraZeneca are "not associated with a decrease in quality of life or worsening of symptoms," the researchers said.
Moderna says vaccine safe, effective in adolescents
Moderna's Covid-19 vaccine was 100% effective in a trial involving 3,732 adolescents aged 12-17, with no major safety problems, the company said on Tuesday. Among participants who received two doses, there were no cases of Covid-19 compared with four cases among those who received a placebo. After only one dose, the vaccine was 93% effective in this age group, Moderna said.
Side effects were similar to those reported in earlier studies, including headache, fatigue, body aches, and chills.
Moderna plans to submit the findings to regulators for emergency use authorization in June. The US Centers for Disease Control and Prevention (CDC) said on Monday it is monitoring rare reports of mild heart inflammation after Covid-19 vaccination in adolescents.
The CDC said the condition is not occurring at higher rates than would be expected in the general population, so no causal link to the vaccine has been established. Dr. Amesh Adalja of the Johns Hopkins Center for Health Security said even if vaccines turn out to be the cause, it is important to consider the risk-benefit ratio.
"Vaccines are going to unequivocally be much more beneficial," outweighing any low risk of myocarditis, he said.