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Unprecedented collaborative efforts in vaccine development have culminated in multiple vaccines being tested in advanced clinical trials all in less than one year since global leaders understood we were in the midst of a global pandemic. One is now being given to health care workers, and another will soon follow.

As the first Covid-19 vaccines are being distributed in the United States and in other countries around the world, the main question now on many minds is, “Are these vaccines safe?”


The answer is yes.

Vaccines are one of the great modern triumphs of public health. They have helped add several decades to human life expectancy and are one of the best tools for preventing disease, debilitation, and death. Immunizations with childhood vaccines prevent 2 to 3 million deaths every year. They are also one of the most thoroughly tested and safest products in history.

We know from recently completed clinical trials of the Pfizer/BioNTech and Moderna vaccines for Covid-19 that serious reactions are rare. And as these vaccines are deployed to millions, we will gain even greater confidence in their safety and effectiveness via post-marketing studies.


However, it’s important to be clear about what “safe” means. No vaccine — indeed, no medical treatment — is completely free from side effects. And it is the responsibility of medical professionals to be honest about them so people are prepared and more likely to trust the science.

Skepticism about vaccines has existed since Edward Jenner first immunized an 8-year-old boy against smallpox in 1796.

The reasons vary from religious beliefs to centuries of medical exploitation inflicted on communities of color and to rampant misinformation on social media. On top of these, Covid-19 vaccines are being developed, tested, and approved at record speed.

Before a vaccine is approved for use by the general public, it must go through a careful process in which it is tested in tens of thousands of volunteers. This system is set up to catch all but the rarest of side effects. Even after a vaccine is licensed, it is subject to stringent safety assessments to detect problems that arise when a vaccine is given to millions of people.

All of that complicates the ability of public health leaders to communicate that side effects and adverse reactions to the vaccine are normal, especially when such reactions become headlines, are amplified on social media, and become fodder for conspiracy theories.

In 2009, for example, the H1N1 vaccine, also known as the swine flu vaccine, was associated with an extremely small risk of Guillain-Barré syndrome, a rare autoimmune disorder that causes nerve damage. Researchers calculated that there were 1.6 extra cases of this syndrome among every 1 million people vaccinated. At the time, the CDC made clear communication a priority, holding near-daily briefings about the country’s vaccination campaign as a way to ease public concerns about the vaccine’s safety. The U.S. had learned its lesson the hard way: In 1976, an overwrought response to a small, contained swine flu outbreak led to many false stories about the side effects of a then newly developed swine flu vaccine.

In other cases, experts have determined that the risks of a particular vaccine were too costly to bear. Take RotaShield, the first vaccine developed to prevent rotavirus, a serious gastrointestinal disease in children. The vaccine was licensed in the U.S. in 1998. A year later, an investigation showed that the vaccine increased the risk of a rare intestinal obstruction by one to two cases per 10,000 infants vaccinated. Vaccinations were halted, and the manufacturer pulled the vaccine from the market.

This experience demonstrates the rigor of America’s vaccine safety and oversight processes and how quickly authorities act if there is a problem. Second-generation rotavirus vaccines were later licensed and deployed, and post-marketing safety evaluations found no increases in risk of intestinal obstruction.

To be sure, experiencing any adverse effect or developing a disease from a vaccine — something that is supposed to prevent disease — can be devastating for individuals and families and should never be taken lightly, even amid a global pandemic. But the overwhelming benefits of vaccines to individuals and society significantly outweigh the risks for adverse reactions.

Thanks to vaccines that make it through the stringent system of testing and approval, infectious diseases that once affected hundreds of thousands of people per year in the U.S. are now exceedingly rare or, in cases, like smallpox, polio, and rubella, are fully eradicated. The downside is that, once a disease is kept at bay by vaccination, we tend to let our guard down and lose sight of how critical vaccines are to keeping them that way.

Covid-19 is yet another instance in which the risk of not being vaccinated is far greater than the risk of side effects posed by the vaccine itself. Of the tens of thousands of people who have already been vaccinated, some have reported short-term symptoms like fever or aches, and a few have reported allergic reactions. Compare that to the virus itself, which has infected more than 70 million people globally and killed approximately 1.6 million — including more than 300,000 in the United States. That’s to say nothing of the devastation caused to economies and health systems around the world.

As the old adage goes, vaccines don’t save lives, vaccinations do. The only way to put a stop to this ongoing tragedy and get back to some semblance of normal is to have widespread vaccination campaigns. That starts with helping people understand — through regular public briefings and mass-media campaigns — that the Covid-19 vaccines are safe while being clear about potential side effects. It also requires complete, ongoing transparency by national, state, and local government and public health officials as more data on these vaccines accumulate.

Only then will we begin to rebuild trust, achieve widespread immunization, and ensure that this virus survives only in our history books.

Wayne C. Koff is the president and CEO of the Human Vaccines Project and adjunct professor, in the Department of Epidemiology at the Harvard T.H. Chan School of Public Health. Michelle A. Williams is the dean of the Harvard T.H. Chan School of Public Health. Both are members of the Human Immunomics Initiative.

  • The RotaShield story is regularly invoked as evidence of the effectiveness of vaccine safety processes in the United States. But in reviewing the details on how its side effects rolled out one would draw an entirely different conclusion. In December 1997 Wyeth presented the results on their RotaShield vaccine to the VRBPAC, just as Pfizer and Moderna just did. The results showed it to be every efficacious (sound familiar?). During the hearings Dr. Carolyn Hardegree, head of vaccine research at the FDA, asked about the cases of intussusception observed during clinical trials. There were 5 cases in the vaccine group and none in the placebo group. Wyeth representative, Dr. Margaret Rennels, stated the incidence was not significant and represented “background rate” or “temporal chance.” The VRBPAC voted unanimously that RotaShield was safe and effective. By March of 1999 there were 3 more cases of intussusception and sixty-two reports of adverse events following RotaShield administration. By June there were 9 more cases of intussusception. When the ACIP met in late June they discussed the findings but made no recommendation. Based upon the MMWR (Morb Mortal Wkly Rep. 1999 Jul 16; 48(27):577-81.) the CDC suspended the use of Rotashield in late July. Not long after the announcement eighty-three more cases of intussusception were reported. A study by Murphy et al (Intussusception among infants given an oral rotavirus vaccine. Murphy TV, Gargiullo PM, Massoudi MS, Nelson DB, Jumaan AO, Okoro CA, Zanardi LR, Setia S, Fair E, LeBaron CW, Wharton M, Livengood JR, Rotavirus Intussusception Investigation Team.
    N Engl J Med. 2001 Feb 22; 344(8):564-72) provided the strongest evidence of an association between the vaccine and intussusception. Murphy’s case-control study compared 429 infants hospitalized with intussusception between November 1, 1998, and June 30, 1999, with 1,763 matched controls. Fifty-two infants required surgery, nine required bowel resection and one infant died. Wyeth pushed back on the safety concerns and the FDA and ACIP eventually shut the door on RotaShield in late 1999.

    The tragedy here and a lesson for us to learn is that the issue of intussusception following RotaShield administration was brought up at the FDA authorization VRBPAC meeting. The early concerns were poo pooed by a representative of the vaccine manufacturer Wyeth. The harm from rotavirus infection, some VRBPAC members said, far outweighed a few cases of intussusception. Wyeth was exempt from liability, just as Moderna and Pfizer are, and was banking on distributing the vaccine world wide. The parallels between RotaShield and the adverse events seen with the Pfizer vaccine raise more than one eyebrow. It begs for full transparency and open reporting to the public. I believe the author of this article is not familiar with the details of how RotaShield was eventually pulled and how the VRBPAC FDA approval process was biased by manufacturer involvement resulting in unnecessary morbidity and mortality for the infants harmed by the vaccine.

  • I want the Covid-19 vaccines to be successful, but there are two things in this article that bother me. The first is that the author himself points out that the initial rotavirus vaccine proved to be unsafe at an unacceptable level after a year-long assessment. That would seem to work against saying that vaccine safety can be established in a matter of months. Is there a reason why Covid-19 vaccines would be different? For the rotavirus vaccine, was it a matter of processing the research over a year or did risks themselves not manifest immediately?

    Second, in comparing the risk of vaccine with risk of Covid-19 on one hand he sites the potential known vaccine side effects with the scale of infection in the pandemic. While that may be a good comparison for an overall population, for an individual you need to compare side effects with the likely effects of Covid-19 for a person with a given health profile. For low-risk people the effects may be comparable. If people nevertheless take it to advance herd immunity that is laudible, but the reason for the choice should be clear.

    Thank you for the many good points in the article.

  • Nice article
    We have friend who tested positive 2 weeks and only developed a cold.
    She is now free of cold symptoms.
    I told her to get retested in a week.
    She said she was told to wait 2 more weeks.
    What should she do?
    I am a cardiac surgeon and have limited knowledge.
    Burt Strug MD

  • It is hard to understand how the authors issue an unequivocal opinion that the current Covid vaccines are safe. These vaccines have been tested for a matter of months and thus the only possible conclusion that can be drawn is that they are likely safe in the short term. There is simply no evidence, however, as to long-term effects.

    Moreover, I do not understand why the authors refer to the success of past vaccines as evidence of safety. The current vaccines do not operate in the same fashion as past vaccines. As reported by one news outlet (and I am not vouching for the accuracy of the description), the current vaccines are different because they use “a new technology (mRNA vaccine)” where “no virus is in the actual vaccine.” The new vaccines operate by inserting an “mRNA strand . . . inside the body’s cells” at which point the body’s “cells use this genetic information to produce this specific antigen on the cell’s surface. This antigen causes the immune system to react, creating immunity. In the case of the Pfizer and BioNTech vaccine, the mRNA is coated with fat and this fat capsule encourages entry into the human cells.” Given that this technology is new, it appears, as a simple matter of logic, that it is not possible to know the long term effects.

    To be clear, I am not opining on whether the new vaccines are safe or whether an individual should take it. I am simply pointing out, at least from what I can glean from this article, that there is no evidence on the long-term effects of these vaccines.

  • This article makes a huge blunder on the number of people infected. In the US alone about 100 million people are already recovered from COVID. The authors confuse “number infected” with the much smaller number of people who have tested positive. Is this blunder intentional to give an inflated impression of mortality rates and to hide the high number of COVID-recovered people who are already likely to be immune and so should wait to be vaccinated? It’s hard to believe these credentialed authors made this blunder by accident.

    • There is literally a comprehensive article dealing with this linked to in the story.

      There were no excess deaths robustly linked to the vaccine. There was however a 2 in one million additional incidence rate of Guillain-Barré syndrome.

    • This Vaccine is not though the trial phase and therefore is still considered an experimental drug. If you are really trying to be transparent, and not seem like there is something to hide, using the statement “it is safe” when the trials are not finished is not helping your argument.

    • Both the Moderna and Pfizer vaccines have completed phase three trials that were far larger (30k and 44k respectively) and more robust than almost any vaccine that has come before so I’d say you should stay away from the keyboard and not write anything about something you obviously have ZERO understanding of.

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