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Scientists at the Food and Drug Administration endorsed the Covid-19 vaccine developed by Moderna as safe and efficacious on Tuesday, one day after the first doses of a competing vaccine from Pfizer and BioNTech were delivered across the United States.

The FDA reviewers said that the two-dose vaccine “was highly effective” in preventing symptomatic Covid-19 from occurring “at least 14 days after the receipt of the second dose.”


Vaccine-related side effects, such as aches and pains, appeared more severe than with the Pfizer/BioNTech vaccine, though such comparisons should be made with caution and are in no way expected to slow the clearance of the vaccine or present major concerns. There was also preliminary evidence that the vaccine has some efficacy after one dose, and that it prevents asymptomatic Covid-19 cases — those that occur without a person ever feeling ill.

Moderna disclosed Nov. 30 that its vaccine decreased symptomatic Covid-19 infections by 94% in clinical trials, while also preventing more severe forms of the disease. But the release of FDA documents provides the most complete look yet at what is likely to be the second Covid-19 vaccine cleared for emergency use against the disease.

The documents were released ahead of a Thursday meeting of outside experts convened by the FDA, the final step before regulators are all but sure to issue an emergency use authorization for the Moderna vaccine. Moderna has asked that its vaccine be authorized for people over 18; the Pfizer/BioNTech vaccine was authorized for people over 16.


At Thursday’s meeting of the Vaccines and Related Biological Products Advisory Committee, the agency will seek the opinions of outside experts who will vote on whether the benefits of the Covid-19 vaccine, mRNA-1273, outweigh its risks. Though the FDA does not have to follow the recommendations of these panels, it usually does.

Moderna previously disclosed clinical trial data showing that 185 cases of Covid-19 occurred in those who received a placebo injection, which was given to half the volunteers in its 30,000-person study. Just 11 cases occurred in those volunteers who received the vaccine.

There were 30 cases of severe Covid-19 in the placebo group of the study, with none in the vaccine group. One volunteer in the placebo group died of Covid-19, compared to none in the vaccine group.

About 10% of volunteers in Moderna’s trial identified as Black, while 20% were Hispanic or Latinx, 5% were Asian, and just under 1% were Native American. In September, Moderna slowed down the enrollment of its trial in an effort to include more participants from underrepresented racial and ethnic groups. 

Like the Pfizer/BioNTech vaccine, mRNA-1273 utilizes a new technology that uses a synthetic version of messenger RNA or mRNA, a key messenger chemical that living things use as part of the process that turn DNA code into the proteins that make up all cells. 

That both of these vaccines use this new technology is a vindication for Moderna, which raised billions of dollars from investors based on the potential for mRNA technology but until now had not delivered a product.

But the Moderna and Pfizer/BioNTech vaccines are not identical. One difference: Moderna’s vaccine tended to cause more vaccine reactions in clinical trials, such as fevers, aches, and chills, than the Pfizer/BioNTech vaccine. 

Severe reactions — which would lead patients to be briefly impaired, but not hospitalized, appear more common with the Moderna vaccine than with the Pfizer/BioNTech one, although comparisons from different trials can be misleading. In the Moderna study, 15.8% of patients had a severe, or grade 3, reaction. Rates of fever and severe fever were similar between studies. In the Moderna study, 65.3% had fatigue, 9.7% of it severe; muscle pain occurred in 58% of patients, 9% severe; joint pain 42.8%, with 5.2% severe.

In the Pfizer/BioNTech study, 59% of volunteers reported fatigue, 4.6% of them severe. Muscle pain occurred in 37.3% of those receiving the Pfizer vaccine. Joint pain occurred in 21.9% of recipients.

However, there were also different side effects between the placebo groups in the two studies. Rates of fatigue were similar, at 23%, in both placebo groups; muscle pain occurred in 12.4% of placebo patients for Moderna and 8% for Pfizer/BioNTech, and joint pain in 10.8% for Moderna and 5.2% in the Pfizer/BioNTech.

These types of reactions are generally expected as a result of the immune response spurred by a vaccine, and are not likely to present an issue to regulators or to doctors administering the vaccine.

FDA reviewers also noted an imbalance in cases of Bell’s palsy, a temporary weakness in muscles in the face, which also occurred with the Pfizer vaccine. There will be monitoring for this side effect. As with the Pfizer/BioNTech vaccine, FDA reviewers noted data that could be an indication of mild allergic reactions. After the Pfizer/BioNTech vaccine started being given in the United Kingdom, there were two reports of severe allergic reactions that had not been seen in clinical trials, leading to increased concern about those side effects.

Moderna said that it has conducted a developmental and reproductive toxicity study, used to assess giving a vaccine in pregnant women, with no adverse effects. Pfizer/BioNTech are still conducting such a study.

One encouraging finding with the Pfizer/BioNTech vaccine was that it appeared possible a single dose of the vaccine was effective in preventing Covid-19. Moderna is presenting a different analysis of the same question, looking at Covid-19 tests that were administered to patients at their first and second doses of the vaccine.

At the second injection, there were 14 cases that tested positive for Covid-19 in the vaccine group, compared to 38 in the placebo group. None of these patients had symptoms. Although the numbers are small (there were more than 14,000 patients in each group) they may indicate that one dose of the vaccine is somewhat effective in preventing the disease, and that the vaccine prevents asymptomatic cases.

  • “The point of healthy people getting the vaccine is so they don’t spread the virus to those that may die from Covid”. The problem is, all these vaccines only promise to relieve symptoms, not prevent the spread. They also say the vaccine may still turn us into asymptomatic carriers, so we can’t stop wearing masks and social distancing. The vaccines also promise us that, after we get the shot, we will most likely get the very same symptoms they are claiming to prevent . All for a flu like illness with a 99.7 % recovery rate. This is not about protecting our health.

  • 11 subjects from the vaccine group developed infection by 14 days after the 2nd dose, and 14 subjects were COVID 19 positive at the time of the 2nd dose.

    That does not add up. By 14 days after the 2nd dose, the number of infections cannot be less than 14

    • So it’s ok that it kills unhealthy people? It’s not all about the individual. If the disease can’t find hosts, it helps to slow/stop the spread, helping to end the pandemic and the burden on the health systems and the economic devastation. It’s also about helping those that aren’t as fortunate to be as healthy as you.

  • I’m a female age 68, Caucasian.
    I’m concerned about taking the vaccine! I take 15 tablets every day plus 5 vitamins! I don’t have a pituitary Gland due to brain surgery 03/31/2010 my head was cut open and had a large tumor that was Benign! Took 30 days of radiation in January 2011: I have had the following:
    My memory is not good
    Broken right ankle compound
    Left knee surgery have 1/2 new knee
    Right knee full replacement
    Broken Right wrist
    Broken left wrist
    Broken left forearm
    I had Bell palsy at age 19
    I hope you can guide me to the right
    Vaccine or should I not take a chance
    At all????

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