Pre-Omicron COVID-19 Transmission 3x Higher Among Unvaccinated

NEW YORK (Reuters Health) – People with breakthrough cases of COVID-19 earlier this year were much less likely than unvaccinated COVID-19 patients to infect others, a population-based study from Germany shows.

The findings are based on data collected until August 2021, before the Omicron variant emerged.

“The number of transmissions from unvaccinated controls was three times higher than from fully vaccinated patients,” Lea Hsu of Public Health Department Cologne and Aachen University and colleagues write in Vaccines. “Fully vaccinated (infected persons) showed a lower viral load, less frequent symptoms, and, above all, a significantly reduced risk of transmission.”

“To effectively end the pandemic, vaccination should be encouraged until herd immunity may be achieved,” the authors advise.

The researchers analyzed Cologne’s health-department data to investigate the risk of transmitting COVID-19 to close contacts. They focused on people living in Cologne who had tested positive for SARS-CoV-2 by PCR and who had been fully vaccinated between December 27, 2020 and August 6, 2021.

Each vaccinated patient was randomly matched by age, sex and virus type (variant of concern or wild type) with an unvaccinated control patient. Patients in the vaccination group were significantly less symptomatic than those in the control group (P<0.001).

The 357 fully vaccinated patients had 979 contact persons, and the 357 unvaccinated matched control patients had 802 contact persons. Of the 979 contact persons of fully vaccinated patients, 99 (10%) became infected; 44 of these were fully vaccinated, 46 were unvaccinated and nine were not fully vaccinated.

By contrast, of the 802 contacts of unvaccinated participants, 303 (38%) became infected; 14 of these were fully vaccinated, 261 unvaccinated and 28 not fully vaccinated.

Factors linked with increased COVID-19 transmission included being unvaccinated (beta=0.237; P<0.001), being male (beta=0.079; P=0.049), and having symptoms (beta = -0.125; P=0.005).

Dr. F. Perry Wilson, an associate professor of medicine at Yale University School of Medicine in New Haven, Connecticut, who was not involved in the study, said, “It’s important to realize that breakthrough infections are uncommon (at least they were at the time of this study), but EVEN in the event of a breakthrough infection, transmission is reduced. That’s good news and suggests, as we know from other studies, that breakthrough infections are milder.”

“Public-health agencies could use these findings to be more lenient on isolation requirements for vaccinated people with breakthrough infections than for unvaccinated infected people,” he told Reuters Health by email. “That might encourage more vaccinations.”

Dr. Brian T. Garibaldi, director of the Johns Hopkins Biocontainment Unit at The Johns Hopkins Hospital in Baltimore, Maryland, who also was not part of the study, told Reuters Health by email, “It is important to understand all of the potential benefits of vaccination. Hopefully these results can help convince unvaccinated individuals to consider getting vaccinated in order to protect not just themselves but those around them.”

“These findings include data only through August 2021, and patients in the study had the Delta variant,” Dr. Garibaldi noted. “Will the same protection from transmitting infection hold true for the Omicron variant?”

The study was not funded, and the authors declared no conflicts of interest.

Hsu did not respond to requests for comment.

SOURCE: Vaccines, online November 2, 2021.

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