New Research Shows The COVID-19 Vaccines May Slow Transmission

Very! Good! News!

by JR Thorpe
Originally Published: 
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You're pining for your vaccine appointments, where you can get your COVID shots in your arm and begin the process of enjoying normal (well, "normal") life again. Evidence has begun to suggest that COVID-19 vaccines can prevent transmission in addition to preventing illness. Research is emerging that the COVID vaccines, whether they’re made of mRNA or an altered cold virus, might indeed slow COVID spread, meaning they might stop people who've had their jab from spreading COVID to others.

The COVID jabs from Pfizer, Moderna, and Johnson & Johnson all work to give your immune system the tools to fight off any virus particles that come your way, and prevent an infection. But no COVID vaccine is 100% protective, so you could get vaccinated and still catch the virus, even if you don't get sick — the same way some people get infected but don’t have symptoms. In theory, this leaves open the possibility that you could then transmit the virus to others, who would be at a greater risk of getting sick if they aren't vaccinated.

Studies now suggest that the presence of vaccine in your body could mean you don't pass COVID on to others. Vaccinated people who end up infected with COVID might only carry a small "viral load" (the amount of virus in their nose and throat), making it a lot harder for them to sneeze, cough, or breathe COVID successfully onto people around them.

Why Reducing Transmission Of COVID Is So Important

Dr. John A. Sellick D.O., a professor at the Jacobs School of Medicine & Biomedical Sciences at the University of Buffalo, tells Bustle that while the most important measure of the COVID vaccines is their ability to prevent severe disease and death, transmission is a big deal, too. If the vaccines stop the COVID virus from breeding in your throat and lungs, you'll carry a lot less of it around, and won't spread it to others.

"While it is unlikely that these vaccines completely stop replication, there is evidence that significant proportions of it can in fact be stopped," Sellick says. "This will lessen the chance that the vaccinated person will shed virus that then can be transmitted to other people." If vaccines completely reduce the risk of spreading the virus, Dr. Kathleen Jordan, M.D., senior VP of medical provider Tia, tells Bustle, then it could be safe for vaccinated people to go to high-risk places like nursing homes, concerts, or sports venues, even before a lot of other people get jabs.

Can Vaccines Stop COVID From Being Spread?

"Early studies suggest vaccines markedly decrease the risk of a person being a carrier or transmitter in addition to preventing illness," Dr. Jordan says. A study of 3,950 U.S. healthcare personnel conducted by the federal government and released by the Centers for Disease Control & Prevention (CDC) on Mar. 29 found that one dose of the Pfizer and Moderna vaccines reduced COVID transmission by 80%, and after two doses, it dropped to 90%. The study looked at people who’d been vaccinated in the real world; it found that out of 2,479 fully vaccinated people, just three — three! — ended up testing positive for COVID.

This confirms earlier data about the mRNA vaccines. One study from Israel, published in a non-peer reviewed study on MedRXiV in February, suggested the Pfizer vaccine in particular could stop transmission in its tracks, reducing the amount of virus people carry around once they've been vaccinated by nearly 90%. Another preliminary UK study in The Lancet published in February, looked at the infection rates among vaccinated healthcare workers and found that the Pfizer vaccine could cut transmission by 86%.

Other COVID vaccines might also reduce transmission. A pre-print study published in The Lancet in February found that people who'd received the AstraZeneca shot, which is not authorized in the U.S., were 50% less likely to receive positive COVID tests, meaning they likely didn't have COVID and couldn't spread it anywhere. But Dr. Carlos del Rio, executive associate dean of Emory School of Medicine, told TIME that those results weren't conclusive. “What they show is that there was decreased viral shedding or decreased detection of virus,” he said — not that the shot definitively stops COVID from being passed on.

"The truth lies somewhere in between, with markedly reduced transmission but still a real risk of being a carrier or spreader post vaccination," Dr. Jordan says. Early signs do point to vaccines helping slow the transmission of COVID, as well as protecting you against it. Experts recommend that you keep masking up and washing your hands until over 80% of the population are vaccinated, though.


Dr. Kathleen Jordan M.D.

Dr. John A. Sellick D.O.

Studies cited:

Dagan, N., Barda, N., Kepten, E., Miron, O., Perchik, S., Katz, M. A., Hernán, M. A., Lipsitch, M., Reis, B., & Balicer, R. D. (2021). BNT162b2 mRNA Covid-19 Vaccine in a Nationwide Mass Vaccination Setting. The New England journal of medicine, 10.1056/NEJMoa2101765. Advance online publication.

Hall, V., Foulkes, S. et al. (2021) Effectiveness of BNT162b2 mRNA Vaccine Against Infection and COVID-19 Vaccine Coverage in Healthcare Workers in England, Multicentre Prospective Cohort Study (the SIREN Study). The Lancet (London, England),

Levine-Tiefenbrun, M., Idan Yelin, I., et al. (2021) Decreased SARS-CoV-2 viral load following vaccination. medRxiv 2021.02.06.21251283; doi:

Thompson, M.G., Burgess, J.L., Naleway. A.L, et al. (2021) Interim Estimates of Vaccine Effectiveness of BNT162b2 and mRNA-1273 COVID-19 Vaccines in Preventing SARS-CoV-2 Infection Among Health Care Personnel, First Responders, and Other Essential and Frontline Workers — Eight U.S. Locations, December 2020–March 2021. MMWR Morb Mortal Wkly Rep. DOI: icon.

Voysey, M., Costa Clemens, S. A., Madhi, S. A., Weckx, L. Y., Folegatti, P. M., Aley, P. K., Angus, B., Baillie, V. L., Barnabas, S. L., Bhorat, Q. E., Bibi, S., Briner, C., Cicconi, P., Clutterbuck, E. A., Collins, A. M., Cutland, C. L., Darton, T. C., Dheda, K., Dold, C., Duncan, C., … Oxford COVID Vaccine Trial Group (2021). Single-dose administration and the influence of the timing of the booster dose on immunogenicity and efficacy of ChAdOx1 nCoV-19 (AZD1222) vaccine: a pooled analysis of four randomised trials. Lancet (London, England), S0140-6736(21)00432-3. Advance online publication.

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