Drugstore Covid-19 tests predict infectiousness; breakthrough risk serious in cancer patients

The following is a summary of some recent studies on Covid-19. They include research that warrants further study to corroborate the findings and that has yet to be certified by peer review.

Drugstore Covid-19 tests predict short-term infectiousness

Drugstore rapid Covid-19 tests, less sensitive than gold-standard PCR tests, might yield a negative result in 15 minutes while failing to detect virus particles but those same particles might pose no risk of transmission in the very short term, according to a report posted on Thursday on medRxiv ahead of peer review.

The researchers first performed these antigen tests on swab samples from 181 individuals with PCR-confirmed SARS-CoV-2 infections.

Next, they took the virus from the swabs and tried to grow it in test tubes. When viral loads were below the antigen tests' level of detection, the virus particles were often incapable of growing.

People with low viral loads and negative antigen tests may become infectious "a day or two or three days later," said Dr James Kirby of Beth Israel Deaconess Medical Centre in Boston.

"Therefore, to be most effective, antigen tests should be used immediately before an event or contact with those at greater risk from infection."

The swabs must be collected for testing carefully, following the instructions provided with the testing kits, he added. "In other words, you want a really good sampling of the inside of your nose."

Breakthrough Covid-19 often severe in cancer patients

Vaccinated people with cancer should not underestimate their risks from breakthrough cases of Covid-19, researchers warn.

Among 54 cancer patients who became infected despite receiving a two-dose vaccine from Moderna or Pfizer/BioNTechor a single-dose vaccine from Johnson & Johnson — before booster doses were recommended — 65 per cent needed to be hospitalised, 19 per cent ended up in intensive care units, and 13 per cent died, according to data from the international Covid-19 and Cancer Consortium.

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The study did not analyse the vaccines' efficacy at preventing infections in the first place.

But among those who did become infected, Covid-19 was no less severe than it was in a comparison group of 1,656 unvaccinated cancer patients with Covid-19, researchers reported on Friday in Annals of Oncology. The risks were greatest for patients with blood cancers.

"Many studies ... have suggested that patients with cancer don't create a strong immune response, and this is the first large study that likely shows the consequences of this," said Dr Jeremy Warner of Vanderbilt University in Nashville, Tennessee.

"Additional doses and boosters are critical, as are continued masking, social distancing, and encouragement of all close contacts of patients with cancer to get vaccinated."