Are mRNA Covid Vaccines Risky? Here’s What the Experts Say

When it became clear in early 2020 that the Sars-CoV-2 virus posed a pandemic threat, researchers who’d been exploring an innovative way to make vaccines saw an opportunity. Experimental messenger RNA vaccines had been recognized for years for their potential speed and flexibility in a fast-moving epidemic, and were some of the first Covid inoculations to move into human trials. The effort paid off when, late last year, vaccines from Moderna Inc. and the Pfizer Inc./BioNTech SE partnership were the first to prove effective. Because the technology is so new, however, these vaccines are particularly vulnerable to disinformation campaigns aimed at dissuading people from taking the shots.

1. How do mRNA vaccines work?

They work in a different way from previous generations of vaccines. Instead of introducing the body to an inactivated or weakened version of a virus or a piece of it, they temporarily turn the body’s cells into tiny vaccine-making factories. They do this using synthesized versions of something called messenger RNA, a molecule that normally carries genetic coding from a cell’s DNA to its protein-making machinery. In this case, the mRNA instructs the body to make the spike protein that Sars-CoV-2 uses to enter cells. This, in turn, stimulates the body to make long-lasting antibodies to the virus. Messenger RNA vaccines are quicker to develop than traditional ones because their production doesn’t require growing viruses or viral proteins inside live cells. Also, mRNA’s modular nature makes designing new vaccines relatively straightforward. It took researchers just a few days in January 2020 to come up with the mRNA sequence used in Moderna’s Covid vaccine.

2. What do we know about their efficacy?

In its phase 3 trial involving more than 30,000 participants, Moderna’s vaccine was 94% effective in preventing symptomatic cases of Covid, according to the company’s filing to the U.S. Food and Drug Administration. There were no cases of severe disease in people who got the vaccine, versus 30 in the placebo group. Pfizer’s vaccine was 95% effective in stopping illness in its phase 3 trial involving more than 43,000 participants, according to results published in the New England Journal of Medicine after the process of peer-review, in which research is scrutinized by experts in the same field. While there were fewer severe cases in that trial, the limited data was consistent with protection against severe disease. Real-world results are just starting to roll in but look good so far. Data from Israel’s largest health-care organization found that after two doses, Pfizer’s shot was 94% effective against symptomatic Covid and prevented 87% of Covid hospitalizations, according to peer-reviewed published results.

3. What do we know about their safety?

Both vaccines can induce strong reactions, especially after the second shot, including headaches, muscle and joint pain, and fever. For example, in the Moderna trial, after the second dose of the shot, most people under 65 experienced fatigue and muscle pain; about half had chills, and 1 in 6 people got a fever. With both vaccines, these side effects are less common in older adults. After the vaccines were authorized for use, officials started noticing rare cases of severe allergic reactions. One theory is that these are caused by the lipid nanoparticles that coat the vaccines, helping carry them into the body. These events are rare, ranging from 2 to 5 cases per million shots given, according to data through Jan. 18 from the U.S. Centers for Disease Control and Prevention. They generally can be treated with adrenaline, also known as epinephrine; a small minority of cases required intubation. A study of data through Feb. 18 from Massachusetts General Hospital suggested a higher rate of severe allergic reactions, about 2.5 cases per 10,000 shots, but still concluded that the overall risk “remains extremely low.” Meanwhile, a separate study from Mass General showed that some people get delayed rashes after getting Moderna’s vaccine. While sometimes dramatic, these rashes are not dangerous, the researchers said.

4. Who’s spreading disinformation about the vaccines?

Traditional anti-vaccine activists have increasingly joined forces with figures on the alt-right, a primarily online political movement based in the U.S. whose members espouse extremist beliefs typically centered on ideas of white nationalism. High-profile conservatives including Fox News personality Tucker Carlson have raised doubts about Covid vaccines generally. According to the U.S. State Department, several online platforms linked to Russian Intelligence have spread disinformation about mRNA vaccines; Moderna and Pfizer are U.S.-based companies. A Kaiser Family Foundation survey found that vaccine refusers are disproportionately reliant on Facebook for their information, whereas people who want the vaccine are more likely to read newspapers or watch network TV news.

5. What are they saying?

  • That steps were skipped in developing and authorizing the vaccines: It’s true that the vaccines reached the market in record time, but that’s not because any testing steps were skipped. Companies sped up the process by performing some testing steps in parallel, and, in the case of Moderna’s vaccine, the U.S. government took financial risks by paying to gear up manufacturing before results were in.
  • The vaccines have never been approved by the FDA: That’s true in the sense that the agency has given only emergency-use authorization so far. This is a pre-existing mechanism created to speed access to medical countermeasures in the event of a public health emergency such as the Covid pandemic. The FDA established in advance that to win authorization, Covid vaccines had to prove to be at least 50% effective in preventing illness in large-scale trials and had to demonstrate safety with two months’ of follow-up data on trial participants. Also, the vaccines were vetted by a panel of independent advisers. Both Moderna and Pfizer have said they plan to apply for regular approval for the vaccines this year.
  • Critics have taken to labeling mRNA vaccines a form of gene therapy, insinuating that the shots might somehow alter your DNA: They don’t. While the messenger RNA they employ is a type of genetic material, the vaccines differ from what is typically thought of as gene therapy in that they do not change the DNA inside cells. “They do not affect or interact with our DNA in any way,” the Centers for Disease Control and Prevention explains. In fact, mRNA molecules in the vaccines, which are short-lived, don’t enter the nucleus of cells, where DNA is stored, the CDC notes.
  • That the lipid nanoparticles in the vaccines may contain antifreeze: That’s not true. Antifreeze contains ethylene glycol, which is toxic. The lipid nanoparticles instead include polyethylene glycol, an inert compound found in everyday products like toothpaste and shampoo and in many drugs including laxatives.
  • That the vaccines can cause antibody-dependent enhancement, or a worse case of disease in those who become sick despite inoculation: This was a theoretical concern when testing of Covid vaccines began. There were hints of this problem in animal studies of some vaccines for severe acute respiratory syndrome (SARS), which is caused by a coronavirus related to SARS-CoV-2. However, no indication of this emerged in human trials of mRNA vaccines for Covid, according to Stanley Perlman, a coronavirus researcher at the University of Iowa, who served on the FDA advisory panel that reviewed the vaccines.
  • That we don’t know the long-term effects of the vaccines: That’s always the case with new vaccines. But vaccine side effects usually show up within the first couple of months after vaccination, which is why the FDA insisted on two months of safety data before authorizing them. Adverse event reports since then have not detected patterns of death that would indicate a problem with the vaccines, the CDC says.
  • That there are more adverse event reports for Covid vaccines than there have been for influenza vaccines: That’s not an appropriate or meaningful comparison, says Aaron Kesselheim, a professor of medicine at Harvard Medical School. The number of these reports tends to surge when a treatment or vaccine is in the news, and nothing has been more in the headlines than Covid vaccines. In the U.S., these reports can be filed by anyone and don’t constitute confirmation that a vaccine caused an adverse event. Given the large numbers being inoculated, some unlucky people will get sick and even die shortly after getting shots, regardless of the vaccine.

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