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Vaccinations for Covid-19 are now underway in the U.S. Two vaccines – one from Pfizer and BioNTech, one from Moderna — have received emergency use authorizations by the Food and Drug Administration. Both vaccines have been deemed safe, but they can cause some short-term side effects.

Vaccines are designed to invoke an immune response that builds protection without causing a serious infection. Traditionally, vaccines are made with viral material which prompts the immune system to mount a defense. This process releases chemicals that cause physical symptoms — such as pyrogens, which are inflammatory chemicals that can cause fever. 

It’s not just vaccines that can cause such a reaction, said Lee Riley, chair of the Division of Infectious Disease and Vaccinology at the University of California, Berkeley. “If you fall down and you get dirt into your skin, some people will develop fever afterwards, if enough immune cells get stimulated.”


The Pfizer and Moderna Covid-19 vaccines are not made with viral material. They are the first vaccines to use messenger RNA technology, known as mRNA, to confer immunity. This essentially adds an extra step. The mRNA has the genetic instructions to create the antigen — which for Covid-19 is the virus’ telltale spike protein. When injected, the body will begin creating this spike protein.

Once the antigen is present in the body, the immune system handles it the same way it would any other invader. That is, the mRNA vaccines don’t cause side effects in a new or different way. 


“The biggest concern is that people might think that the vaccine could actually give them Covid-19,” said Matthew Laurens, professor at the Center for Vaccine Development and Global Health at the University of Maryland School of Medicine. “It absolutely can’t, especially with these mRNA vaccine products that only contain the genetic code.” He stressed that educating the public is crucial, so when people experience common side effects they will know that it just means the vaccine is working.

  • Alex,

    You did not mention at all the risk, however small, for severe allergic reaction or ansphylaxis. Thus, your article IMO is incomplete and inadquate! Care to respond?

  • My 93 year old mother had the Phizer vaccine 5 days ago (uk). Adverse mild symptoms of increased temperature, headache and feeling tired day 2 to day 4.

  • Im concerned about the moderna vaccine triggering Bells Palsy , as i had long lasting Bells Palsy as a teen, and am in the next group to be vaccinated- 75+. There have been no further comments about this side effect, would i not be able to get it? I heard 2 cases on the same day, but have heard nothing since. Can you elaborate and give any further info??

    • The chances of developing bells is lower than the average occurrence in the general population without the vaccine.

  • As a health care provider, up to date with all my vaccines, I feel like I see lots of information addressing short term side effects (normal, like mild symptoms, sore arm etc) which I am not particularly concerned about. What I don’t see addressed adequately is the unknown long term effects of mRNA vaccines. For example in an interview with Dr Offit (who has made a career in promoting and developing vaccines)” regarding these mRNA vaccines:

    Dr. Offit: More concerned. We have no commercial experience with them. The vaccines are naked pieces of mRNA encapsulated in a complex lipid delivery system that enables the cell to take it up. If you injected the mRNA straight, ribonucleases (enzymes that dismantle RNA) would dissolve it. Once in the cell, the mRNA begins self-reproducing. It’s now making the coronavirus spike protein, which, for the most part, gets inserted into the cell membrane and to a lesser extent into the circulation.

    The foreign proteins will be broken up into 15 to 20 “mers” (pieces) and placed on the cell so helper and cytotoxic T cells see them. The mers are also on antigen-presenting cells (which alert the immune system), and myocytes (muscle cells) too, perhaps just the ones that are damaged from the injection. We don’t know exactly.

    What turns it off? In mice it goes on for 10 days. In humans, I don’t know. We’ll find that out.

  • Your body responds to the particles it creates using the info from the vaccine. This is like a recipe, and it is only a knock-off of a tiny, tiny part of the covid virus. It’s not real, it is just a string of proteins that are created by man that appear to your body to be foreign material, which wake up your body’s immune system. The four signs that your immune system is woken up are pain, fever, swelling, and redness. We think it is the germ that causes this response, but it is our body’s attempt to make our bodies a threatening environment to the germ, and to kill it off. People who have immune disorders can’t kill off the germs. They just keeps multiplying until they kill us.

    The whole virus will make you sick. This knock-off can’t. It’s like a sentence, or a word, and the virus is a book. It’s all that other stuff in the virus that try to take over our bodies ability to kill it off, so they can multiply.

    The virus doesn’t exist in the vaccine like it did in the past…this is only a party of it. It is the part of the virus that hooks into our cells and starts budding off thousands of virus infected cells using the machinery within our own cells to multiply. Viruses must have a host to replicate, they can’t do it by themselves, and they must use our cell machinery to do it.

    But the good thing about our immune system is it recognizes a threat, and will gear up fever or whatever it takes to kill off the germ. That little man made piece that mimics the virus looks exactly like the hook that the virus uses to enter our cells.

    You may get sick because your body is gearing up to kill the threat (the man-made code). That’s our immune systems reaction, not the germ. You will get swelling, fever, redness, or pain because you have a working immune system. The thing about the immune system is if you tick it off, it is very spiteful, it never forgets. The next time it sees something like that man-made invader, it sends in armies of immune cells to kill it off before it gets a chance to hook into your machinery.

    As long as the man-made string of proteins look like the real virus’s hook…it will attack quickly, before it gets a chance to breed. Immune systems recognize foreign invaders, and do so quickly once they have been introduced. The vaccine introduces them in a new way, and it works.

    It might not be exactly a new way, science has been working on this way for over 20 years, if I remember correctly…but it hasn’t been used before. That makes it a little frightening. And there are still people who can be allergic to the fat molecules (polyethelene glycol) that carry the coded proteins into the body. They are susceptible, their bodies see the fat molecules as invaders, where most of us don’t…just like egg allergies to vaccines. Their immune systems go crazy trying to kill off the invader. Doctors and health providers KNOW this, and can treat them. This is why you need to stick around for a bit, until the likelihood of your body’s immune system going on overdrive is past, anywhere from 15 minutes to one-half hour. It’s very serious business. We haven’t always known this, but now we do. About one in 25,000 people have had this reaction, but health providers aren’t taking a chance.

    • It has been over 30 years since I had a seize is it ok foe me and I’m allergic to red die the medience use on me back in 87 face turned red.

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