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Has there been any further research on asymptomatic transmission, especially in conjunction with the new variants?

Note: No Q&A tomorrow.

Question: Has there been any further research on asymptomatic transmission, especially in conjunction with the new variants?

Answer: It has been a while since we revisited the research on asymptomatic transmission and there has been a lot of research published!  CDC’s current best estimate is that: a) 40% of cases are asymptomatic; b) asymptomatic individuals are 75% as likely to transmit infection as symptomatic individuals; and c) 50% of new cases are the result of asymptomatic infection.  When it comes to asymptomatic transmission and new variants, we still need more research.  Do the new variants cause more asymptomatic and/or pre-symptomatic infection?  A different period of infectivity?  Currently, we have some evidence indicating that those who are infected with B.1.1.7 (UK variant) are more likely to experience any symptoms (see Q&A of 2/16), which would indicate a lower proportion of asymptomatic infection.  The evidence base, however, is too shallow to draw any firm conclusion yet.  What is abundantly clear thanks to a substantial and growing evidence-base is that asymptomatic transmission is real, it is an important driver of the pandemic, and to control the pandemic we have to limit asymptomatic transmission (hello, universal masking!).  Included herein are a few more details.   

  • A recent meta-analysis published earlier this year in Annals of Internal Medicine examined results of 61 eligible studies to understand the proportion of cases that remain asymptomatic.  Researchers concluded that at least 33% of SARS-CoV-2 infections are asymptomatic.
  • Results from modeling asymptomatic infection published earlier this year in JAMA found that “59% of all transmission came from asymptomatic transmission, comprising 35% from presymptomatic individuals and 24% from individuals who never develop symptoms. Under a broad range of values for each of these assumptions, at least 50% of new SARS-CoV-2 infections was estimated to have originated from exposure to individuals with infection but without symptoms.”
  • A recent systematic review published earlier this year in Lancet Microbe found that “Several studies reported similar viral loads at the start of infection among asymptomatic and symptomatic patients infected with SARS-CoV-2; however, most studies demonstrated faster viral clearance in asymptomatic individuals, as also seen in MERS-CoV, suggesting a shorter infectious period but with similar potential transmissibility at the onset of infection.”
  • A study of asymptomatic infection in healthcare workers published last year in Clinical Microbiology and Infection found that among healthcare workers who became infected with SARS-CoV-2, those who consistently used N95 respirators and wore eye protection were more likely to be asymptomatically infected, indicating a dose/response relationship to viral exposure and severity of symptoms.