Vaccinations & Asymptomatic Transmission of SARS-CoV-2

Breakthrough Infections

Recent studies show that a small percentage of fully vaccinated persons have developed symptomatic or asymptomatic COVID-19 breakthrough infections1-7. These individuals received the recommended doses of an FDA-authorized COVID-19 vaccine at least two weeks before they were infected with SARS-CoV-2. According to the CDC, 10,262 SARS-CoV-2 vaccine breakthrough infections had been reported from 46 U.S. states and territories as of April 30, 20218. The CDC reported this number is likely a substantial undercount because those with breakthrough infections who are asymptomatic or who experience mild illness, might not get tested.

Vaccinated People Can Be Asymptomatic Carriers

Preclinical studies of COVID-19 vaccines demonstrated persistent virus in nasal swabs even after being vaccinated. During the trial of Moderna’s vaccine, researchers saw a significant drop but not elimination in the number of asymptomatic infections among people who received the first shot of the two-dose vaccine, compared with those who received a placebo9. During a trial of the AstraZeneca vaccine, researchers swabbed participants every week, and estimated a 49.3% reduction in asymptomatic infections among a subset of vaccinated participants compared with the unvaccinated group10. This data suggests people can have asymptomatic infections even after being vaccinated.

Viral Load in Vaccinated Individuals

With other variants, researchers noted a significant drop in viral load in people who were infected with SARS-CoV-2 in two to four weeks after receiving their first dose of the Pfizer vaccine, compared with those infected in the first two weeks after the injection11. The AstraZeneca trial also observed a larger reduction in viral load in a group of vaccinated participants than in the unvaccinated group12. On July 27th the CDC announced that people infected with the Delta variant can have high viral loads and are infecting others even after being fully vaccinated15.

Vaccinations and asymptomatic transmission of SARS-CoV-2

Low Limits of Detection Critical for Detection of Samples with Low Viral Loads

If the viral load of a sample is lower than the LOD or cut-off value of a real-time RT-PCR assay, a false negative result will be obtained. An assay with low LOD (high sensitivity) is critical for accurate detection of samples with low viral load.

Vaccinations Reduce, but Do Not Eliminate the Chance of Viral Spread

CDC states that while the risks of SARS-CoV-2 infection in fully vaccinated individuals and transmission to others is reduced, it cannot be eliminated in the setting of continued widespread community transmission of the virus. Vaccinated people could potentially still become infected and spread the virus to others13.

Testing Vaccinated Individuals for Asymptomatic Infections

Since some vaccinated individuals have a reduced viral load, a highly sensitive test with a low LOD must be used to prevent the reporting of false negatives13. The FDA developed an experiment to precisely compare the performance of the nucleic acid-based SARS-CoV-2 assays which have received EUA authorization and published a comparative performance analysis14. This assessment used the FDA SARS-CoV-2 Reference Panel which allowed a consistent determination of the relative sensitivity of these tests. See how the assays rank in comparison to each other. Recently obtained Next Generation Sequencing (NGS) data on vaccinated individuals who tested positive with the PerkinElmer New Coronavirus Nucleic Acid Detection Kit provided orthogonal confirmation that the virus was present in these individuals.


  1. Baden LR, El Sahly HM, Essink B, et al. (2021) COVE Study Group. Efficacy and safety of the mRNA-1273 SARS-CoV-2 vaccine. N Engl J Med 2021;384:403–16. 10.1056/NEJMoa2035389.
  2. Sadoff J, Gray G, Vandebosch A, et al. (2021) ENSEMBLE Study Group. Safety and efficacy of single-dose Ad26.COV2.S vaccine against COVID-19. N Engl J Med 2021;NEJMoa2101544. 10.1056/NEJMoa2101544
  3. Thompson MG, Burgess JL, Naleway AL, et al.(2021) 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. MMWR Morb Mortal Wkly Rep 2021;70:495–500. 10.15585/mmwr.mm7013e3
  4. Tenforde MW, Olson SM, Self WH, et al. (2021) IVY Network; HAIVEN Investigators. Effectiveness of Pfizer-BioNTech and Moderna vaccines against COVID-19 among hospitalized adults aged ≥65 years—United States, January–March 2021. MMWR Morb Mortal Wkly Rep 2021;70:674–9. 10.15585/mmwr.mm7018e1
  5. Tande AJ, Pollock BD, Shah ND, et al. (2021) Impact of the COVID-19 vaccine on asymptomatic infection among patients undergoing pre-procedural COVID-19 molecular screening. Clin Infect Dis 2021;ciab229. Epub March 10, 2021. 10.1093/cid/ciab229 [PMC free article] [PubMed] [CrossRef] [Google Scholar]
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  7. Haas EJ, Angulo FJ, McLaughlin JM, et al. (2021) Impact and effectiveness of mRNA BNT162b2 vaccine against SARS-CoV-2 infections and COVID-19 cases, hospitalisations, and deaths following a nationwide vaccination campaign in Israel: an observational study using national surveillance data. Lancet 2021;397:1819–29. 10.1016/S0140-6736(21)00947-8.
  8. CDC COVID-19 Vaccine Breakthrough Case Investigations Team (2021). COVID-19 Vaccine Breakthrough Infections Reported to CDC – United States, January 1-April 30, 2021. MMWR. Morbidity and mortality weekly report, 70(21), 792–793.
  9. Food and Drug Administration. Moderna COVID-19 Vaccine. Vaccines and Related Biological Products Advisory Committee December 17, 2020 Meeting Briefing Document Addendum- Sponsor. 
  10. AstraZeneca. COVID-19 Vaccine AstraZeneca confirms 100% protection against severe disease, hospitalisation and death in the primary analysis of Phase III trials
  11. Levine-Tiefenbrun, M., Yelin, I., Katz, R. et al.Initial report of decreased SARS-CoV-2 viral load after inoculation with the BNT162b2 vaccine. Nat Med (2021). 
  12. Oxford says COVID-19 vaccine with AstraZeneca works against UK variant.
  13. CDC. Science Brief: Background Rationale and Evidence for Public Health Recommendations for Fully Vaccinated People
  14. FDA Reference Panel Comparative Data. April 6, 2021.
  15. Pettypiece, S., Przybyla, H., Strickler, L. and Tirrell, M. (July 27, 2021, 9:16 AM CDT) CDC recommends the vaccinated wear masks in areas with low vaccination rates.

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