Study shows SARS-CoV-2 breakthrough infections 18-fold lower than rate of infections among unvaccinated

Vaccine breakthrough infections (VBTs) occur in individuals, even after they have been vaccinated against a particular pathogen. With the radical increase in the number of coronavirus disease (COVID-19) cases caused by the Delta variant of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the United States of America, understanding the rate and clinical features associated with VBTs among those already vaccinated is of critical public health importance.

In a recent study published on the preprint server medRxiv*, researchers conducted a pilot study to assess the prevalence and features of VBTs through the use of sequencing and case data on Washoe County, Nevada available from the Washoe County Health District (WCHD) and Nevada State Public Health Laboratory (NSPHL).

Study: An Analysis of SARS-CoV-2 Vaccine Breakthrough Infections and Associated Clinical Outcomes. Image Credit: David Pereiras /

Analysis of public health data from WCHD

In the current study, the researchers collected all available public health data from February 12, 2021, to July 29, 2021. A total of 6,399 new cases of COVID-19 were reported during this time, of which 6,128 (95.8%) were successfully traced back to their sources. Out of these 6,399 cases, the Health District identified 339 (5.5%) as breakthrough infections.

VBT cases were defined as any positive reverse-transcriptase polymerase chain reaction (RT-PCR) test for individuals fully vaccinated for 14 days or more, prior to symptom onset or routine testing. The WCHD collected individual-level VBT data for 338 cases out of the 339 traced.

Recorded parameters on these VBTs included demographics such as age, sex, gender, and race/ethnicity, as well as clinical data such as vaccine type and dosage schedule, symptom onset; hospitalization or death. Information on cycle threshold (Ct) values and variants of interest, primarily from the NSPHL and from other partner laboratories when necessary were also collected.

VBT accounted for a small share of total infections between February 12 and July 29, 2021. The 338 reported VBT cases accounted for only 0.14% among Washoe’s vaccinated population across the study period, in comparison with a 2.54% rate among those unvaccinated.

Severe outcomes in vaccinated individuals were also rare. Out of the 5,961 traced cases, 289 (86%) vaccinated individuals presented with symptomatic infections, but only 17 were hospitalized (5%).

No deaths were reported in the cases of VBT, and the cases of hospitalizations involved mostly elderly people. However, elderly people were less prone to breakthrough infections and more likely to present without symptoms compared to younger age groups.

Study implications

The current analysis from Washoe County, Nevada showed a more than 18-fold lower rate of breakthrough infections among vaccinated people over the rate of infections among unvaccinated individuals. Most importantly, the rates of severe illness were low in vaccinated individuals.

These findings stress the efficacy of vaccines in preventing severe infections and hospitalizations. There is already a growing body of data that can reassure the public on the real-world effectiveness of COVID-19 vaccines.

However, it should be noted that the data discussed here is part of a pilot dataset used for analyzing the outcomes of vaccinations. Nonetheless, this analysis adds to existing literature outlining the risks of VBTs and emphasizes the need for health authorities to monitor evolving trends in vaccine breakthrough data.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information

Journal reference:
Sreetama Dutt

Written by

Sreetama Dutt

Sreetama Dutt has completed her B.Tech. in Biotechnology from SRM University in Chennai, India and holds an M.Sc. in Medical Microbiology from the University of Manchester, UK. Initially decided upon building her career in laboratory-based research, medical writing and communications happened to catch her when she least expected it. Of course, nothing is a coincidence.


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  1. Doug McCoy Doug McCoy United States says:

    Of course the data shows more un-jabbed infections. The jabs were not rolled out until mid-December 2020. So there were very few fully jabbed people by 2-12-2021. They did not ramp up giving the jab until April of 2021. Plus considering people who tested positive prior to 14 days after they had received their 2nd jab as "unvaccinated" also skewed the results. This "study" is meaningless.

  2. Atilda Mosham Atilda Mosham Australia says:

    We hear about the breakthrough cases to be lower in the vaccinated than the number of infections in the unvaccinated. This however is a very little satisfying information since UK Government official document "SARS-CoV-2 variants of concern and variants under investigation in England Technical briefing 20" clearly demonstrated that those fully vaccinated are 5.3 times more likely to die than the unvaccinated when inf3ected with the Delta variant. Why we don't hear about that?

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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