Immune response after mild Covid-19 is prolonged; psychiatric risk revealed

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.

Mild Covid-19 induces prolonged immune response

In patients with mild Covid-19, immune responses last months and possibly longer, researchers found. Early reports suggested that in mildly ill patients, antibodies decrease and immunity wanes soon after recovery.

But a study from China last month on 349 Covid-19 patients, which has not yet undergone peer review, found similar immune response patterns at six months regardless of symptom severity.

And in a study published on Saturday ahead of peer review, US researchers performed blood tests in 15 patients after mild Covid-19, looking for three signs of lasting immune responses: antibodies, so-called memory B cells, and memory T cells.

Three months after recovery, patients still had "all three of these defense layers," reducing their risk of reinfection, study co-author Lauren Rodda of the University of Washington School of Medicine told Reuters. If they do become reinfected, they are less likely to become severely ill or be contagious, Rodda said.

Test results at three months were unchanged from results at one month, so her team believes this is a lasting response. Because the findings show immune responses last three months, if not longer, Rodda added, they support US Centres for Disease Control and Prevention advice that patients need not be retested for Covid-19 within the first three months after an infection.

Both studies were posted on the website medRxiv.

Covid-19 survivors at risk for psychiatric disorders

A study of more than 62,000 Covid-19 survivors has found significant risks for mental health issues. Researchers found that one in 16 Covid-19 patients who never had a mental illness will be diagnosed with one within three months after infection.

This risk is about twice as high as expected and is even higher among patients who were sick enough to be hospitalised, study leader Maxime Taquet of the University of Oxford told Reuters. Most common are anxiety disorders, but depression, insomnia, and rarely, dementia, also occur, he said.

The study, reported on Sunday on the medRxiv website ahead of peer review, also found higher-than-average Covid-19 rates in people with a previous psychiatric diagnosis. Taquet's advice for patients: "If you experience anxiety, low mood, insomnia or memory loss after Covid-19, you should see a medical professional as there might be ways to improve these symptoms."

Viral load not linked with smell or taste recovery

Virus levels in the nose and throat have been linked to Covid-19 symptom severity, but Hong Kong researchers who expected viral load to correlate with smell and taste impairment were in for a surprise: viral load was not linked to the severity of these so-called olfactory and gustatory symptoms, nor with how long it takes for the sense of smell or taste to return to normal.

The findings, reported in the journal Laryngoscope, are based on data from 39 patients in Hong Kong who developed problems smelling or tasting - or both. On average, it took 10 days for these senses to return.

Four to six weeks after becoming ill, 72 per cent had completely recovered the ability to smell and 83 per cent were able to taste again.

But there was no statistically significant link between viral load and severity of these symptoms or the recovery time.

Early use of antibody-rich convalescent plasma may be best

Treating seriously ill Covid-19 patients with antibody-rich blood plasma from people who have recovered from the disease can lower the risk of death, and new data from a nationwide US study may help fine-tune the use of this so-called convalescent plasma.

At 2,807 hospitals between April 4 and July 4, more than 35,000 hospitalised patients with, or at risk of, life-threatening Covid-19 respiratory problems received a transfusion of at least one unit of Covid-19 convalescent plasma.

Roughly half the patients were in intensive care units and roughly one-quarter needed mechanical ventilators. Mortality rates were lower when plasma was given within three days of diagnosis, rather than later, the researchers found.

And the more antibodies in the plasma, the lower the recipients' risk of death.

In a report posted ahead of peer review on medRxiv, the research team concluded that while the study was not a gold-standard randomized trial, the findings added to evidence that "the quality and manner in which convalescent plasma is administered to patients hospitalised with Covid-19 may reduce mortality."

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