During July 2021, 469 cases of COVID-19 infections associated with densely packed indoor and outdoor events that included bars, restaurants, guest houses and rental homes in a town in Barnstable County, Massachusetts, were reported. Approximately, 3/4 of cases (346, 74%) occurred in those who were fully vaccinated (Brown CM, Vostok J, Johnson H, et al. Outbreak of SARS-CoV-2 infections, including COVID-19 breakthrough infections, associated with large public gatherings—Barnstable County, Massachusetts, July 2021. MMWR Morb Mortal Wkly Rep. 2021 Aug 6;70(31):1059-1062.

Genomic sequencing confirmed that the dominant circulating strain was the delta variant of SARS-CoV-2. Overall, 274 (79%) of infections were mildly symptomatic with the most common symptoms being cough, headache, sore throat, myalgia and fever. Four of 346 previously vaccinated individuals (1.2%) were hospitalized (two of whom had underlying medical conditions). In addition, using PCR testing and cycle times, the authors concluded that the quantity of virus shed was similar between vaccinated and unvaccinated groups who were subsequently infected.

As a result of these findings, the Centers for Disease Control and Prevention (CDC) recommended that all persons, including those who are fully vaccinated, should wear masks in indoor public settings in areas where COVID-19 transmission is high or substantial.

There are several takeaways from this report:

  • First, consistent with national statistics, only 1.2% of those who were previously vaccinated were hospitalized, confirming that vaccination is highly effective at preventing severe/critical infection.
  • Second, while vaccines that prevent “mucosal” infections like SARS-CoV-2 are often excellent at preventing severe/critical disease, they are not as highly effective at preventing asymptomatic or mildly symptomatic infections. It is, therefore, unreasonable to call these asymptomatic or mildly symptomatic infections “breakthroughs” — a term that should be reserved for those who despite full vaccination suffer severe or critical infections.
  • Third, investigators chose to study “viral load” by examining PCR and cycle times at only one point in time. Because PCR detects quantities of nucleic acids, a sample with less virus requires higher cycle times to detect the virus. However, other investigators using this method have demonstrated that while the levels of antigen appear similar in samples from vaccinated and unvaccinated people, the time curves differ in that quantities of antigen drop off more quickly in vaccinated people as the memory immune response is activated. (Chia PY, Ong SWX, Chiew CJ, et al. Virological and serological kinetics of SARS-CoV-2 delta variant vaccine-breakthrough infections: a multi-center cohort study. medRxiv preprint doi: https://doi.org/10.1101/2021.07.28.21261295). Therefore, it is incorrect to state that vaccinated people who are subsequently infected shed the same amount of virus as those who were not previously vaccinated.

Materials in this section are updated as new information and vaccines become available. The Vaccine Education Center staff regularly reviews materials for accuracy.

You should not consider the information in this site to be specific, professional medical advice for your personal health or for your family's personal health. You should not use it to replace any relationship with a physician or other qualified healthcare professional. For medical concerns, including decisions about vaccinations, medications and other treatments, you should always consult your physician or, in serious cases, seek immediate assistance from emergency personnel.