The authors analyzed 1,373 persons with Covid-19 in the San Francisco Bay Area from February 1 to June 30, 2021. 125 of them had vaccine breakthrough infections.
They found that symptomatic breakthrough infections had similar viral loads to symptomatic unvaccinated infections, and were over-represented by antibody-resistant SARS-CoV-2 variants, as you can see in the graphic, Figure A, at right. Moreover, they found that those with breakthrough infections were just as contagious as those who were unvaccinated with infections.
NOTE: In the graphic above (click to enlarge) look for the Delta variant and see how much larger it is in the vaccinated. Indeed, the unvaccinated appear to be filtering it out.
The authors noted that the percentage of breakthrough cases increased from 0% to 31.8% during the 5-month period of the study, as the vaccination rate increased. They also found that the vaccinated were more likely to be carrying the more infectious variants, as you can see in Figure B.
QUOTE: “The predominance of immune-evading variants among post-vaccination cases indicates selective pressure for immune-resistant variants locally over time in the vaccinated population concurrent with ongoing viral circulation in the community.”
Moreover, the authors found that the viral loads of symptomatic cases were about equal between vaccine breakthrough cases and unvaccinated cases, and their viral load was 445 times higher than asymptomatic cases (those without symptoms but happened to have a positive PCR test).
The authors recommend that whether or not they are vaccinated, symptomatic people should be wearing masks, but asymptomatic people are not likely to be contagious.
NOTE: The authors describe one patient who had Covid, then got the JnJ vaccine, and then had a breakthrough case. They suggested that the JnJ vaccine may not be very potent against the Delta variant, or that having had the infection did not confer much protection, but they did not discuss the possibility of the vaccine interaction with natural immunity. While it is claimed that vaccinating after having had Covid will “top up” immunity, this has not been proven over any long-term period.
QUOTE: “In summary, our results reveal that selection pressure in a jhighly vaccinated community (>70% fully vaccinated as of early August 2021) favors vaccine breakthrough infections from antibody-resistant VOCs (vaccines of concern) such as the Gamma and Delta variants, and that high-titer symptomatic post-vaccination infections may be a key contributor to viral spread.