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Staff Photo by Matt Hamilton / Pharmacist Jessica Duke takes inventory of the different varieties of vaccine she has available during a block party at the BlueCross Healthy Place at Highland Park in Chattanooga on Saturday, June 26, 2021.

The rise of the more contagious delta variant of the coronavirus and preliminary data have ignited discussions around whether mixing different COVID-19 vaccines could boost their effectiveness.

"There is some fragmentary data — and I would like to emphasize fragmentary — that the Johnson & Johnson vaccine may not be quite as effective as the Moderna and the Pfizer vaccine in preventing serious disease caused by the delta variant," said Dr. William Schaffner, a professor of infectious diseases at Vanderbilt University Medical Center. "I still think they're in the same ballpark, although there may be a little gap between them. But that has raised the question about whether people who've received the J&J vaccine should receive a follow-up dose."

U.S. residents now have access to three COVID-19 vaccines — the two-dose messenger RNA (mRNA) vaccines, Pfizer and Moderna, and the single-dose viral vector Johnson & Johnson vaccine.

Much of the mixing-vaccines discussion and limited, preliminary research at this time is emerging from Europe — where the AstraZeneca vaccine, also a viral vector vaccine like the Johnson & Johnson vaccine, is widely used.

With the exception that the AstraZeneca vaccine requires two doses compared to Johnson & Johnson's single dose, the vaccines are quite similar, Shaffner said, and some European countries are now encouraging people who have gotten one dose of AstraZeneca to get a second dose of an mRNA vaccine, such as Pfizer or Moderna, despite limited data as to whether mixing vaccines is safe and effective.

The U.S. Centers for Disease Control and Prevention's vaccine advisory committee now states that the COVID-19 vaccines are "not interchangeable."

Diana Kreider, director of clinical services at the Hamilton County Health Department, said the department follows CDC guidance and requires that people adhere to the drug manufacturers' dosing schedules.

The department monitors which vaccines an individual has received through the Tennessee Immunization Information System (TennIIS) database.

"If a patient states that they are there to receive a second dose vaccination, we check their vaccination record card for the date and product type of their first vaccination. If they do not have their card, we are able to access their information via TennIIS," Kreider said via email.

Schaffner said European countries tend to be less conservative in their recommendations compared to the CDC. But if convincing data around mixing vaccines emerges, it would likely facilitate discussions here at the CDC Advisory Committee, he said.

"It's an area which has not been carefully, rigorously, comprehensively studied," he said. "So, at the moment, there's not much there for the CDC's advisory committee to look at, even though there's a lot of interest in the question."

(READ MORE: COVID-19 'smolders' in the Chattanooga region's unvaccinated population)

In the United States, the public health emphasis is not on fine tuning the already highly effective vaccines but getting more people in general to get vaccinated, Schaffner said.

"We've vaccinated the eager beavers. They've come forward and received their vaccines. It's now a lot of work for absolutely every additional person [to get] vaccinated, particularly in our part of the country," he said. "I'm concerned and distressed and disappointed by all that. We need to get more people vaccinated, because this virus is not going to disappear."

Contact Elizabeth Fite at efite@timesfreepress.com or follow her on Twitter @ecfite.

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