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A new study suggests the messenger RNA vaccines produced by Moderna and the Pfizer-BioNTech partnership appeared to be 90% effective in preventing Covid-19 infection in a real-world setting.

The study was released Monday in Morbidity and Mortality Weekly Report, an online journal published by the Centers for Disease Control and Prevention.


The study followed nearly 4,000 health care workers, first responders, and other essential workers in eight U.S. locations as the first Covid vaccines were rolled out starting in December. Participants were tested weekly to look for all cases of Covid infection, even asymptomatic ones.

In the period from Dec. 14, 2020, to March 13, 2021, nearly 75% of the workers in the cohort received at least one dose of one of the mRNA vaccines. Both are given in a two-dose schedule.

There were 161 Covid infections in the unvaccinated workers, compared with 16 in workers who had received only one dose by the time of their infection and only three infections in people who had received both doses and were two weeks out from their second dose. The vaccine effectiveness following two doses was 90% — roughly in line with the 95% and 94% that the Pfizer and Moderna vaccines showed, respectively, in the clinical trials that supported their emergency use authorizations.


The study was not conducted in such a way as to allow the researchers to estimate effectiveness measures for each of the vaccines.

The study suggested that even the first dose of vaccine was 80% effective at preventing infection, starting from two weeks after receipt of that dose. That estimate, however, only applies to the brief period until the second dose was administered. The study was not designed to test how well the vaccine works if an individual does not receive the second dose.

The majority of infections — 58% — detected in the study were found because participants were tested weekly to look for infections; 42% of the infections were identified when study participants were tested after developing symptoms. Most of the people who tested positive in the study had some Covid-related symptoms, though 10.7% had none. Only 23% of the people who became infected needed medical care and only two were hospitalized. There were no Covid deaths in the study.

    • All COVID-19 vaccines currently being administered in the U.S. have been shown to be safe with no issues regarding blood clotting. Of course, with a history of stroke, any other medical history or in generality it is wise to consult one’s primary care physician before inoculation.

    • The incidence of blood clots in people (with no previous history) is statistically higher in people who have not received the vaccine than people who have. Don’t fall prey to the logical fallacy of “post hoc, ergo procter hoc” – which translates to “after, therefore because of.” In other words, just because someone suffers a blood clot FOLLOWING a vaccine, does not mean that it was CAUSED BY the vaccine. If that reasoning was applied, the statistics I referred to above would “prove” that the vaccine PREVENTS blood clots. More likely, one has absolutely nothing to do with the other – as hundreds of thousands of peer-reviewed studies (thanks to brave volunteers) have proven. Please think these things through before you put yourself (and others) at risk based on what you’ve “heard.” Misinformation is as infectious as the Virus itself. Best of luck to you!

  • Interesting article, but the story wrapped up with a final paragraph I found left it unclear whether the infection figures were among all participants or only those vaccinated (which is what I want to know about.) 16 infections occurred in the one-dose cohort but the final paragraph says 23% of infected persons in the study needed medical care. That would be 3.6 people if referring to the vaccinated group. So the 23% figure must include non-vaccinated persons, too? What about those who received two doses? What percentage needed medical care if they became infected?

  • I have completed my vaccine 3 weeks ago. My first was okay no reactions, when I got my 2nd dose 3 days after my skin rashes suddenly appeared and spread out so quick all over my body. They were red bumps like mosquito bites and really itchy. I started to worry that it might continue and lead me to serious problem. Fortunately, thanks God. the itchiness and spreading stopped on the 3rd day. And many of people I knew telling me it’s normal reaction and I’ll be okay soon.. They were right.. Thanks God.. I’m now feeling fine rashes were gone.

  • I am still concerned getting this vaccine.I have COPD and Moderate to Severe Asthma.I had Covid in November and was a moderate one.I know people who got it and all had different or no reactions.So am I gonna be ok if I get this?And so people who have had Covid have a less reaction to the vaccine if you already had Covid?Please help me cause tomorrow I am scheduled and don’t know what to do..

    • Hey Mark! I have long-term lung damage from a valley fever infection. I got my first dose of Moderna on March 23rd. So far, I haven’t had any issues except for a little dizziness and feeling a bit off, and typical arm pain after a shot. I’m on Advair 100/50, Proair albuterol, Singulair, and Zyrtec for my issues. If you’re super worried about it, I’d say talk to your doctor. I was nervous, too, but after three days or so I was back to running my farm like I always do.

    • Mark, I have diffuse bronchiectasis, and got my 1st vaccine this past Saturday. I felt great both Sat and Sun, but then started feeling very tired on Sun evening. So Mon I felt tired and just a little nauseous. I woke up this morning and feel almost back to normal.

      Get the vaccine!! If you contract this virus with lung disease, you are going to be in trouble! And if you are fortunate enough to make it through alive without being vaccinated, you’re going to lose lung function. I know the 2nd vaccine will likely bring on more side effects than the 1st, but I’ll take feeling a little rough for a couple days if it means that I’m going to be 90 – 95% protected from severe disease.

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