Delta Knocked Down Pfizer Vaccine’s Effect on Transmission

Pfizer’s COVID-19 vaccine (Comirnaty) showed a decreased ability to blunt transmission of the Delta versus Alpha variant, and its effect on transmission also appeared to wane over time, U.K. researchers found.

Following the second dose of Pfizer vaccine, vaccine-associated decreases in transmission of the Delta variant were smaller than decreases in transmission of the Alpha variant (adjusted rate ratio [aRR] 1.59, 95% CI 1.07-2.35), reported David Eyre, DPhil, of the University of Oxford in England, and colleagues.

While transmission of the Delta variant decreased by 50% 2 weeks after the second dose of Pfizer, that declined to a 24% (95% CI 20-28) reduction in transmission 12 weeks after the second dose, they wrote in the New England Journal of Medicine.

These declines were similar, if not more substantial, in those who received two doses of the AstraZeneca vaccine, the researchers found.

Eyre’s group noted that viral loads in vaccinated people with the Alpha variant were lower, but with Delta, viral loads were similar in vaccinated and unvaccinated individuals.

This “calls into question whether vaccination controls the spread of the Delta variant as effectively […] and whether, with increased transmissibility, the maintained viral load after vaccination explains the rapid global spread of the Delta variant despite increasing vaccination coverage,” the authors wrote.

They examined contact-tracing data from 146,243 contacts of 108,498 index cases in England from Jan. 2 to Aug. 2, 2021. Of these, 54,667 (37%) tested positive via PCR. Median age of index patients was 34, and 51% were women, while median age of contacts was 43 and 57% were women. Two-thirds of exposures were in households and residences, with 11% apiece at visits to households, events and activities, and workplace or educational facilities.

Overall, 46% of contacts of unvaccinated index patients tested positive via PCR, as did 21% of contacts of index patients who were fully vaccinated with Pfizer’s vaccine and 28% of contacts of those fully vaccinated with AstraZeneca’s vaccine.

Median time from second vaccination to a positive PCR test among individuals fully vaccinated with the Pfizer vaccine was 90 days and 51 days with AstraZeneca.

In addition, multivariable models found the Delta variant was associated with “more onward transmission” from symptomatic cases (aRR with a contact age of 18 years 1.24, 95% CI 1.12-1.38) and asymptomatic cases (aRR with a contact age of 18 years 1.40, 95% CI 1.22 to 1.59).

“The delta variant eroded vaccine-associated protection against transmission both by making infection more common and by increasing transmission from infected vaccinated persons,” the authors said.

Interestingly, Eyre’s group noted that variations in cycle threshold (Ct) values in index patients at diagnosis only explained 7% to 23% of the effect of vaccination.

“This finding indicates that Ct values measured in diagnostic testing are not necessarily a surrogate for the effect of vaccination on transmission,” they wrote.

Limitations to the data included only those contacts with a positive PCR test, meaning that “absolute protective effects of vaccination on transmission may be underestimated because vaccine-protected, uninfected contacts may not have sought testing,” they wrote.

Contacts may have also been infected by a source other than the index patient, and there may have been potential misclassifications of the Alpha and Delta variant, as S-gene target failure and time was used as a proxy for genomic sequencing. Also, there was no adjustment for preexisting conditions.

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    Molly Walker is deputy managing editor and covers infectious diseases for MedPage Today. She is a 2020 J2 Achievement Award winner for her COVID-19 coverage. Follow


Eyre disclosed support from Gilead Sciences, National Institute for Health Research (NIHR), and the Robertson Foundation.

Co-authors disclosed support from Qatar National Research Fund, Department of Health and Social Care, Huo Foundation and NIHR.

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