Limited data suggest that not only are the mRNA Covid-19 vaccines effective in reducing the risk of symptomatic disease  by greater than 90% but also reducing the risk of asymptomatic infections by 80-90% after the second dose and by 62-80% after the 1^st dose. ^1-3

A CDC study of health care personnel, first responders, and other essential and frontline workers who received one of the 2 currently available mRNA vaccines (BNT162b2-Pfizer or mRNA-1273-Moderna) and underwent weekly testing for SARS-CoV-2 infection irrespective of symptoms found 90% efficacy in preventing infection among fully immunized (≥14 days after 2^nd dose) and 80% efficacy in preventing infection among the partially immunized (≥14 days after 1^st dose).  The majority of SARS-CoV-2 infections were identified by weekly specimens, with 11% remaining without symptoms.¹

In a retrospective study of over 39,000 asymptomatic adult patients undergoing pre-procedural SARS-CoV-2 molecular screening tests, an 80% reduction in the risk of a positive test  was observed in those who had received 2 doses of an mRNA vaccine (majority Pfizer) and 72% reduction in those following a single dose of vaccine >10 days prior to their pre-procedure test.²  In the original Moderna trial , a 62% reduction in the risk of asymptomatic infection was seen among participants just before the second dose (ie, partially immunized).³ 

Collectively, these reports support the high efficacy of mRNA vaccines in reducing the risk of SARS-CoV-2 in asymptomatic infection.  Whether these findings can be reproduced with other vaccine preparations is not known at this time!

Bonus Pearl: Did you know that according to 1 study, asymptomatic patients with SARS-CoV-2 infection may be more likely to be women, younger and have shorter duration of viral shedding? ⁴

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References

1. Thompson MG, Burgess JL, Naleway AL, et al. Interim estimates of vaccine effectiveness of BNT162b2 and mRNA-1273 Covid-19 vaccines in preventing SARS-CoV-2 infection among health care personnel, first responders, and other essential and frontline workers—Eight U.S. locations, December 2020-March 2021. https://www.cdc.gov/mmwr/volumes/70/wr/mm7013e3.htm
2. Tande AJ, Pollock BD, Shah ND, et al. Impact of the Covid-19 vaccine on asymptomatic infection among patients undergoing pre-procedural Covid-19 molecular screening. Clin Infect Dis 2021. https://pubmed.ncbi.nlm.nih.gov/33704435/
3. Baden LR, El Sahly HM, Essink B, et al. Efficacy and safety of the mRNA-1273 SARS-CoV-2 vaccine. N Engl J Med 2021;384:403-16. https://www.nejm.org/doi/full/10.1056/nejmoa2035389
4. Yang R, Gui X, Xiong Y. Comparison of clinical characteristics of patients with asymptomatic vs symptomatic coronavirus disease 2019 in Wuhan, China. JAMA Network Open 2020; May 27. https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2766237

Disclosures: The listed questions and answers are solely the responsibility of the author and do not necessarily represent the official views of Mercy Hospital-St. Louis or its affiliate healthcare centers, Mass General Hospital, Harvard Medical School or its affiliated institutions. Although every effort has been made to provide accurate information, the author is far from being perfect. The reader is urged to verify the content of the material with other sources as deemed appropriate and exercise clinical judgment in the interpretation and application of the information provided herein. No responsibility for an adverse outcome or guarantees for a favorable clinical result is assumed by the author. Thank you!