Weekly Update: December 31st, 2020 - January 5th, 2021

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Welcome to the McGraw-Hill’s COVID-19 channel. This week we have information on the sensitivity of rapid antigen testing versus PCR testing, new CDC allergy contraindications to being vaccinated, and an update on the more infectious variant of SARS-CoV-2.

If you think they have COVID-19 and the antigen test is discordant with the patient’s symptoms, consider doing a PCR. This is a study of SARS-CoV-2 detection using the Sofia SARS Antigen Fluorescent Immunoassay (FIA) compared to real-time reverse transcription–polymerase chain reaction (RT-PCR) which was considered the gold standard. A total of 1098 paired were tested.

  • In those who were asymptomatic (871), the antigen test was 41% sensitive, 98% specific with a positive predictive value of 33% negative predictive value of 99%.
  • In those who were symptomatic (227) the antigen test was 80% sensitive with a specificity of 99% and a negative predictive value of 96%.

Antigen testing is a rapid and less expensive alternative for screening for COVID-19. However, compared to the gold-standard RT-PCR, the performance characteristics lag. What does this mean? A positive antigen test in an asymptomatic patient is likely a false positive. Do a PCR. A negative antigen test in a symptomatic patient is likely a false negative. Do a PCR.

If your clinical judgment disagrees with the antigen test, get an RT-PCR on that patient. The full paper can be found here.

  • Pray IW, Ford L, Cole D, et al. Performance of an Antigen-Based Test for Asymptomatic and Symptomatic SARS-CoV-2 Testing at Two University Campuses — Wisconsin, September–October 2020. MMWR Morb Mortal Wkly Rep 2021;69:1642–1647. DOI: http://dx.doi.org/10.15585/mmwr.mm695152a3external icon

The FDA has updated the allergy contraindications to the SARS-CoV-2 vaccine (verbatim). "The CDC considers a history of the following to be a contraindication to vaccination with both the Pfizer-BioNTech and Moderna COVID-19 vaccines:

  • Severe allergic reaction (e.g., anaphylaxis) after a previous dose of an mRNA COVID-19 vaccine or any of its components
  • Immediate allergic reaction of any severity to a previous dose of an mRNA COVID-19 vaccine or any of its components (including polyethylene glycol [PEG])*
  • Immediate allergic reaction of any severity to polysorbate (due to potential cross-reactive hypersensitivity with the vaccine ingredient PEG)*

* These persons should not receive mRNA COVID-19 vaccination at this time unless they have been evaluated by an allergist-immunologist and it is determined that the person can safely receive the vaccine (e.g., under observation, in a setting with advanced medical care available)."

Other points of note: The vaccine should be offered regardless of a history of prior symptomatic or asymptomatic infection. Patients who are currently symptomatic can still be vaccinated but it should be deferred until the patient is asymptomatic and safe to leave isolation. A complete discussion of the vaccine indications/contraindications can be found here.

According to NPR reporting, the new, more infectious, SARS-CoV-2 variant has been found in Colorado, California and Florida. The Canadian Broadcast Corporation notes that it has also been found in Canada (Ontario, British Columbia, Alberta and Quebec) as well as South Africa and elsewhere. According to the CDC, there is no evidence that it causes worse disease. There is no anticipated effect of the new UK variant on the effectiveness of any vaccine. Studies are ongoing.  A transcript of the CDC press briefing can be found here. There is some concern about the South African variant and current vaccine effectiveness.

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