Nearly 4,000 Frontline Workers Studied for mRNA Vaccine Efficacy. The Results? It’s Working.

Sujal Mandavia, MD
April 6, 2021
6 min

In the world of vaccine development, there are trials and studies in controlled settings, but real-world situations serve as the ultimate proving ground. The two messenger RNA, or mRNA, vaccines against COVID-19, produced by Moderna and Pfizer-BioNTech, have now been put to the test in the real world — and the results so far have been impressive.

For decades, scientists have seen the promise of mRNA, but it has only now been utilized in a significant way. Early trials showed the vaccines could be 94 percent (Moderna) and 95 percent (Pfizer) effective in preventing COVID-19 infection. Those were controlled studies, and since December the hope has been that the vaccines could offer similar results in the real world. One recent research report from the CDC (Centers for Disease Control and Prevention) involving nearly 4,000 people indicates that these vaccines are indeed delivering on their promise. Here’s a look at what the CDC found.

About the Research

Let’s first look at how these mRNA vaccines work. Often referred to as the Pfizer and Moderna vaccines, they operate as messengers, giving instructions to our cells to produce a “spike protein” identical to one found on the surface of the SARS-CoV2 virus, which causes COVID-19. This action encourages our body to develop antibodies to fight that spike protein and build our immune response to the virus. (For full efficacy, both vaccines require two doses, the manufacturers have emphasized.)

The real-world research involving the mRNA vaccines was originally published on March 29, 2021, in the CDC Morbidity and Mortality Weekly Report (MMWR), an online journal of the CDC. In its research, the CDC tracked a cohort of 3,950 people. Subjects were first responders, healthcare workers, and other essential workers in eight facilities around the country. The sites were in Miami; Salt Lake City; Phoenix and Tucson, Ariz.; Duluth, Minn.; Portland, Ore.; and Temple, Texas.

The research was conducted between the months of December 2020 (the start of the vaccine rollout) and March 2021. Everyone in the cohort was monitored regularly for symptoms, and each person completed weekly real-time polymerase chain reaction (RT PCR) tests for SARS-CoV-2 for 13 straight weeks. Of those studied, 2,479 (62.8%) had received a full vaccination, or two doses, and 477 (12.1%) had received only one dose and were considered partially immunized. The rest were unvaccinated.

This study was particularly helpful because of the weekly surveillance testing. Some other studies have looked only at patient-reported symptoms, but we know that asymptomatic infections have contributed to COVID-19’s spread during the pandemic. This study would have identified those, too, so it doesn’t have that vulnerability.

The Results

The mRNA vaccines were shown to be highly effective in preventing symptomatic SARS-CoV-2. In fact, the data revealed 90 percent effectiveness in preventing SARS-CoV-2 in those who were fully immunized, regardless of symptom level, and 80% effectiveness in those who were partially vaccinated.

The unvaccinated group showed 1.38 SARS-CoV-2 infections per 1,000 person-days, whereas fully vaccinated people had only 0.04 infections per 1,000 person-days. The partially immunized group showed 0.19 infections per 1,000 person-days. These findings are in line with both vaccines’ phase 3 trials, which indicated efficacy above 90 percent for those who had both doses.

The CDC hasn’t completely determined which vaccine was more effective. At this time, it estimates that about 62.7 percent of people who were fully vaccinated received the Pfizer vaccine and 29.6 percent received the Moderna vaccine. About 7.7 percent of the vaccinated population are “pending product verification,” according to the report.

“These findings indicate that authorized mRNA COVID-19 vaccines are effective for preventing SARS-CoV-2 infection, regardless of symptom status, among working-age adults in real-world conditions. COVID-19 vaccination is recommended for all eligible persons,” the CDC said in its report.

What the Research Tells Us

There are still many questions to explore, including how long immunity lasts and the impact of having recovered from COVID-19 on the immunity of an unvaccinated person. But this research is an important first step in demonstrating that these vaccines are indeed working at a very high level. In time, more data can be collected to offer even more proof. This can build confidence in people who may be hesitant about getting the vaccine right now. When that happens, we take another important step toward our goal of herd immunity or at least having a great degree of control over the spread of the virus.

As more vaccinations are administered, count on the CDC to continue to produce more data on critical real-world efficacy. It’s important information we all need.

Do you like what you’ve read? If you want to learn more about Carbon Health or other health issues, download the Carbon Health app or visit carbonhealth.com.


Carbon Health’s medical content is reviewed and approved by healthcare professionals before it is published. But note that our knowledge and understanding of COVID-19 are developing and changing very rapidly; if you have questions or concerns about COVID-19 precautions, treatments, and vaccinations, please talk to your healthcare provider.


Sujal Mandavia, MD

Dr. Mandavia has over 20 years of healthcare management experience. In his role as Chief Medical Officer at Carbon Health, Mandavia oversees all clinical operations and protocols, as well as the expansion of Carbon Health's world-class clinical care offering. When not at work, Sujal enjoys everything hockey related, listening to the Tragically Hip, and being taken for hikes by his Rhodesian Ridgebacks.

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