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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.

  • I have a question. If I took a Coricidin HBP cold & cough pill 8 to 9 hrs before my first Covid-19 vaccination could it lessen the antibodies to build up in my immune system? Or, the vaccination effacy rate of the Moderna for the 1st shot?

  • I had the 1st dose of vaccine yesterday, initially I didn’t feel a thing, but last night I had severe aches locally and generally, fever but the worst is the chills, I had Tylenol twice to be able to sleep!! Is that expected??? Thanks

    • I have given quite a few Moderna and Pfizer vaccines and I believe the side effects of the shot depends on how strong the recipient’s immune system is. Similar to giving the high dose flu shot to the elderly, most I have spoken to had minimal side effects. I wonder if their dose needs to be increased?

  • Muy buenas Dios los bendiga por todo ese apollo por las personas
    Yo fuy apollado por el govierno de los estsdos unidos y la organisacion panamericana de la salud con una donasion y me mandaron a presentar unas ordenes de las vacunas para colombia contratos vilaterales diretamente con el laboratorio de pfizer
    Dios los bendiga

  • I had some very minor covid symptoms two weeks after first moderna vaccine, and tested positive 24 days after that shot. Should I follow through and get second dose as scheduled?

  • I have several questions. There is a still a 10% chance to get COVID if you have 2 shots, has there been any evidence that the COVID is minor if you do catch it. The second question is do you have to get a booster shot every year? Thanks Dennis

    • I would say that it is minor imo. The vaccine is still going to fight the virus. Like it says in the last paragraph of the article, “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”. (copied and pasted from above)

    • Will be every year. There’s not enough profit for the drug companies with just a one vaccine deal. You may say but it’s free. Nothing is free in this world. I will not be receiving the vaccine and encourage other young healthy adults not to either.

    • The amazing thing is that we have these vaccines which will help us open up again and no one understands the numbers. When Pfizer says their vaccine is 95% effective, that means it’s that you have that much better chance at not getting symptomatic Covid then if you don’t get a vaccine which is already pretty low and not that 5% of people will still get Covid. So you basically have a 20x chance of not getting Covid than if you don’t get a vaccine. This is better than Rubella vaccine and way better than flu vaccine. More importantly all these companies have stated in their clinical trials (and the numbers have been reviewed and approved by the various agencies) that the vaccines are 100% effective against severe Covid symptoms and death from Covid. This article would have been a lot more useful had they actually categorized who got Covid, which cohort went to the hospital or died, etc.

  • “…only two were hospitalized.” I’m assuming that’s two out of the entire 177 infected participants. How many of the 19 vaccinated yet infected (“breakthrough”) participants required hospitalization? How many of the breakthroughs were symptomatic? How many breakthroughs required medical attention?

    • it depends on your immune system. Generally speaking, no. At least, as long as the virus doesn’t mutate enough to escape the mounted immune response, and your immune system works regularly. Upon being infected, and recovering from the virus, you became naturally immunized against it. The vaccine shot then should have acted as a booster shot to cement the ability to fight the virus and hopefully have created a lasting (longer than a year) immune memory.

  • Denmark observational study with 370K participants shows quite different numbers than CDC observational study with 4K participants

    medRxiv article on vaccine effectiveness VE for BioNTech vaccine in Denmark, for 39,040 LTCF (long-term care facilities) residents (median age 84 years) and 331,039 frontline healthcare workers HCW (median age 47 years)

    — no protective effect was observed for LTCF residents after first dose.
    — In HCW, VE was 17% in the > 14 days after first dose (before second dose).
    — at day 0-7 of second dose VE in LTCF residents was 52% and 46% in HCW.
    — Beyond seven days of second dose, VE was 64% for LTCF residents and 90% for HCW.

    • Alex,

      I would argue that this Denmark result corroborates with, or at least not sufficient to dispute CDC’s study. First, the low efficacy in LTCF residents with a median age of 84 is not surprising. Prominent immunosenescence is inevitable in people at this age. In other words, their immune systems are so aged that they are not competent to mount good immunity when vaccinated.

      Booster shots (the second shot) are critical for sustained immunity formation. Without booster shot, the first shot is able to prime the immune system but not sufficient to form sustained immunity. This is how our immune system is designed. Therefore, if you look at your last bullet point, beyond 7 days of second shot, the efficacy in LTCF is 64%, which I would argue pretty darn good considering their ages, and 90% in HCW (median age 47), which corroborates with CDC’s data.

    • To clarify, the primary comparison on my above post was for the vaccine effectiveness VE at least 14 days after first but before second dose:

      • VE=17% in Denmark study 331K participants
      • VE=80% in CDC study 4K participants (see study abstract)

      Both science and common sense indicate that VE calculated using observational data from study of 331K study is much closer to reality than using data from study of 4K participants

      17% in 331K study versus 80% in 4K study

    • Results from different retrospective studies are hard to compare because you cannot account for all the differences between groups but I think this reinforces the need to get both doses. Just got my first one today.

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