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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.

  • Were the participants exposed intentionally to the virus. If not, how can the study be valid. It is not a controlled study, otherwise. Their environment must be controlled,also so all factors are equal.

    • Hi! It’s a fair question. The controls in this case were the unvaccinated healthcare workers. his particular study was intended to “real-world”, which basically means you know you can’t control or document everything. If the vaccines weren’t effective you’d expect similar numbers of infections in vaccinated and unvaccinated. However, as the article above explains, there were 160 infections among unvaccinated versus 16 in vaccinated groups. That’s a 90% reduction and strong evidence for the efficacy of the vaccine.

      To put that another way, how do you explain the 90% reduction in infections in the vaccinated group if it WAS NOT due to the vaccine?

    • Are you saying that the general public is intentionally exposed to the virus? The people receiving the placebo are the control group. Everything else about the people is the same. Intentionally exposing them would be a different type of trial.

    • Yes.

      There is evidence that the approved vaccines prevent severe COVID-19 to a high degree. There is not evidence that having been previously infected with COVID-19 prevents reinfection or severe COVID-19 to the same degree.

  • Some casual readers may be confused by the 90% effectiveness number, which (while good) seems lower than previously-stated 95% numbers. It should be noted that the 90% vaccine effectiveness is an adjusted number which reflects the wide confidence interval caused by so few fully-vaccinated healthcare workers who tested positive.

    Only three fully-vaccinated people during 78,902 person-days tested positive. Since asymptomatic people were proactively tested, some of those three could have been asymptomatic.

    By contrast 161 non-vaccinated people during 116,657 person-days tested positive. Superficially that appears vaccinated workers in those conditions are 1/35th as likely to test positive, hence you might naively assume the vaccine was 97% effective.

    However such few positive cases (which by itself is good) mandates a statistical adjustment that results in the 90% number. It’s like determining average height of 10,000 people by only measuring three of them. You are less confident the small sample size of three reflects the average height of 10,000, and that is expressed in a confidence number.

    IOW the mRNA vaccines are so effective this makes it difficult to measure exactly how effective they are. The value of this study was the real-world nature, outside of a Phase III trial. The results don’t necessarily mean the mRNA vaccines are “only” 90% effective — rather 90% is the only conclusion supported by the tiny number of fully-vaccinated people who tested positive in that group. Future broader studies will refine this. In general the mRNA vaccines seem nearly 100% effective at preventing severe symptoms, hospitalization and death, but that also will be adjusted with more precision over time.

  • All of the users making these wild claims about the COVID-19 vaccines. It’s not true. They’re Trumpsters scaring people so they don’t get the prevention they need.

    Do not watch Fox News. They lie to you. If they were to be fined for every lie they broadcast, they would be out of business in no time at all.

    Do your own research. You can go to your local library and ask how to access the research databases. You can find out anything you want to know. I do mean anything.

    If you choose to go with what you heard, or what you see online, you may die by suicide (if you don’t get a shot).

    I think this disease would make you consider preventing it anyway you could.

    If you make negative claims about effective measures we have to treat this virus, you may influence someone who is going to die. Their deaths are on you.

    So, site your “professional” sources. It must be a credible source of information. No Fox News.

  • In this study, being healthcare workers, is it safe to assume they were all masked?

    I would be interested to see the results of a study that researches the effectiveness without masks.

    • I am a woman 82 years old. Due to a lasting adverse effect ( so far two months swelling under eyes) doctors have advised against second dose…How effective is one dose? for how long? what alternatives do i have if one dose is not enough to protect me?When can i see others that have had two doses without mask?…i have asked this last question online to at least five doctors, the only answer being wait and see… please respond. thank you ellen

  • I have searched the internet for more data regarding long term possible effects of the Covid vaccines. I have found no data. There is loads of information suggesting efficacy but no other information that I am aware of.
    I am not anti vaccines but I am hesitant to get the vaccine that is not FDA approved. I would like to know more. Can anyone direct me to sources that provide more information? I can not seem to find any in the mainstream media.
    I had Covid the first week of March 2020 when there was no mask wearing or lockdown. I had antibodies until at least December 2020. But was retested twice last week. They are untraceable at this point.

    • I recommend listening to This Week in Virology podcasts which consist of virologists, immunologists and clinicians hashing out the science with assorted scientists on the cutting edge of the different research. The rna vaccines are very simple, don’t even have any preservatives. I was convinced despite early reservations.

    • ” I have searched the internet for more data regarding long term possible effects of the Covid vaccines. I have found no data. ”

      The vaccines are new. How would there be long term data ?

  • What the study doesn’t do is show what will happen to those people down the road which is why the FDA has not approved it. They approved an unapproved product for emergency use but Clearly have not given their stamp of Approval!

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