Immune system has long-term defences after mild Covid-19; children may be key carriers of virus variants

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.

Immune system may never forget mild Covid-19

Months after recovery from mild Covid-19, when antibody levels in the blood have declined, immune cells in bone marrow remain ready to pump out new antibodies against the coronavirus, researchers reported on Monday (May 24) in Nature.

Upon infection, short-lived immune cells are generated quickly to secrete an early wave of protective antibodies. As the immune cells die out, antibody levels decline.

But a pool of these immune cells, called long-lived plasma cells, is held in reserve after infection. Most of them migrate to the bone marrow, explained coauthor Ali Ellebedy of Washington University School of Medicine in St. Louis.

His team obtained bone marrow samples from 19 patients seven months after the onset of mild Covid-19. Fifteen had long-lived plasma cells secreting antibodies against the coronavirus.

Five of the 15 had second bone marrow biopsies 11 months after symptom onset and all still had long-lived plasma cells secreting antibodies against SARS-CoV-2.

Ellebedy, in a statement, noted that these cells are "just sitting in the bone marrow and secreting antibodies. They have been doing that ever since the infection resolved, and they will continue doing that indefinitely... These cells will live and produce antibodies for the rest of people's lives." It is not clear yet whether the same results would be seen in survivors of moderate to severe Covid-19, the authors said. 

Coronavirus variants seen more often in young US children

Young children appear to be significant carriers of more contagious variants of the new coronavirus, such as the ones identified in the UK and in California, according to a new US study.

From March 2020 to April 2021, researchers at nine children's hospitals tested a total of 2,119 Covid-19 patients age 18 or younger for so-called variants of concern, along with key mutations that help the variants become more contagious or hide from the patient's immune system.

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"In total, we identified 560 of these important mutations, and 75 per cent (420/560) ... were in children less than 12 years of age, the population that is currently not eligible for Covid-19 vaccines," said Jennifer Dien Bard of Children's Hospital Los Angeles, lead author of a report posted on Monday on medRxiv ahead of peer review.

The results provide "clear evidence" of the emergence of variants of concern in pediatric patients across diverse geographies and socioeconomic populations in the United States, the researchers said in their paper.

They added that the results highlight the need for ongoing monitoring of pediatric populations "who will be among the last groups to receive vaccination and who are key to ending this pandemic." 

Dentures may harbor more bacteria during Covid-19

Denture wearers infected with the coronavirus may be more likely to harbor bacteria that increase their risk for additional infections on top of Covid-19, a small study suggests. Researchers analysed removable dentures from 30 patients with Covid-19 and 30 healthy individuals.

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Swabbing the surface of the dentures yielded Streptococcus species in 93.3 per cent of Covid-19 patients versus 40 per cent of healthy volunteers, and Klebsiella pneumonia bacteria in 46.7 per cent of Covid-19 patients versus 13.4 per cent of the other group.

All of these microorganisms are seen as potentially important causes of additional bacterial infections in hospitalised Covid-19 patients that could lead to or worsen problems such as respiratory distress syndrome and pneumonia, the researchers reported on Friday in The Journal of Prosthetic Dentistry.

They recommend that dental prostheses in Covid-19 patients be tested and that appropriate medications be used to prevent the invasion of bacteria in vulnerable individuals.