This week's roundup of some of the latest scientific studies on the novel coronavirus and efforts to find treatments for COVID-19 recognize obstacles in the vaccine race to immunize the globe, especially when it comes to cancer patients, while mapping out the gateways of the coronavirus and dispelling myths about vitamins.

mRNA vaccines may need more study for treating some cancer patients

COVID-19 vaccines that employ new messenger RNA (mRNA) technology to fend off the illness may require further testing before it becomes clear they are safe for cancer patients with solid tumors, cancer treatment experts say. That would include the vaccines from Pfizer Inc. with partner BioNTech SE and from Moderna Inc.

The vaccines work by carrying genetic instructions for making a protein from the virus that the immune system learns to recognize and defend against. The mRNA is encased in a microscopic "packet" called a lipid nanoparticle (LNP), or liposome, that protects and delivers the fragile molecule into cells.

While this technique is new for vaccines, it has long been used to carry cancer drugs into tumors, because tumors easily take up liposomes. This raises the possibility that tumor tissues would capture some of the liposomes carrying the vaccine's mRNA, said Gerard Milano of Nice University in France.

The potential for part of the vaccine to become stuck in tumors and the resulting effect on cancer patients "is an open question to which there is currently no answer," Milano said.

In a paper published in the British Journal of Cancer, his team calls for "a careful evaluation of the efficacy of these promising mRNA COVID-19 vaccines administered as lipid carriers for patients with solid tumors, including a possible re-appraisal of the dosing for optimal protection of this specific and frail population."

New coronavirus crosses membranes between throat and brain

The coronavirus uses the nose as a gateway into the brain, autopsy findings suggest. The presence of the virus in the brain and cerebrospinal fluid has been linked with neurological symptoms, but exactly how the virus enters the central nervous system has not been clear.

During autopsies of 33 patients who died of COVID-19, researchers examined the nasopharynx – the region where the nasal cavity connects with the back of the throat – which happens to be in close proximity to the brain. By dissecting cells and following the path of infection, they saw that the virus invades the brain by crossing through the mucous membranes that separate it from the nasopharynx.

From there, according to a report published in Nature Neuroscience, it may travel along the nerve fibers that connect the nasal cavity to the part of the brain involved in the sense of smell, which would explain "some of the well-documented neurological symptoms in COVID-19, including alterations of smell and taste perception."

The researchers also found virus particles in brain regions with no direct connection to the nose, which suggests there may be additional routes of viral entry into the brain.

Vitamin D fails to help in severe COVID-19 cases

Low levels of vitamin D have been linked to a higher risk for severe COVID-19, but high vitamin D levels do not fix the problem.

Increasing vitamin D levels in critically ill patients did not shorten their hospital stay or lower their odds of being moved to intensive care, needing mechanical ventilation or dying, doctors in Brazil found.

They randomly gave 240 patients hospitalized with severe COVID-19 either a single high dose of vitamin D3 or a placebo. Only 6.7% of patients in the vitamin D group had "deficient" levels of the nutrient compared with 51.5% of patients in the placebo group, but there was no difference in the outcomes, according to a paper posted on medRxiv ahead of peer review. The same was true when the researchers focused on 116 patients with vitamin D deficiency before the treatment.

The authors say theirs is the first randomized trial of its kind to show that vitamin D supplementation "is ineffective to improve hospital length of stay or any other clinical outcomes among hospitalized patients with severe COVID-19."

Immune system responds strongly in asymptomatic COVID-19

Asymptomatic people infected with COVID-19 are mounting robust immune responses that differ from responses in those who become ill, according to a study that appears to contradict previous thinking by health experts.

Researchers studied immune system T cells in 76 symptomatic COVID-19 patients and 85 infected individuals without symptoms and reported their findings on bioRxiv ahead of peer review.

Some of these cells – CD8+ T cells – can recognize virus-infected cells and destroy them. They also produce inflammatory proteins, or cytokines, that help to prevent the virus from making copies of itself. Others known as CD4 helper T cells help the body produce B cells, which make antibodies.

Everyone in the study had similar frequencies of T cells that could recognize the virus and destroy infected cells, regardless of whether they had symptoms. But the T cells of asymptomatic individuals produced higher levels of cytokines important for fighting the virus, including interferon-gamma and interleukin-2.

"What we still need to understand is whether those T cells can persist over time and offer long-term immunity," said co-author Antonio Bertoletti of Duke-NUS Medical School in Singapore.