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This article was originally published in "Out of the Bench", a science communication blog I maintained, in July 2020, during the height of the COVID-19 pandemic, and after the first vaccines started to be released.

We have been bombared with news telling us it's possible to be fully vaccinated and still catch COVID-19 and that it's possible to get serious side effects from the vaccine. This constant bombarding of what can happen after getting the shot can place doubts in the mind of the most sober-minded of individuals.

Can I catch COVID-19 after taking the vaccine?

At first, the benefits seem obvious: one should get vaccinated to keep one from catching COVID-19, but the reality is a bit more complex than that.

Vaccines do offer very good protection against COVID-19, but, as we have been seeing in international news, there have been cases in which, even though someone was vaccinated, they still got infected with the virus. That is undoubtly a true possibility but, in scientific speech, "being possible" simply implies that the likelyhood of happening, as low as it is, is not zero.

To understand why this happens, we have to mind how much our bodies are different from each other. The vaccine accomplishes its task of making your imune response quicker and more efficient (as I mentioned in previous articles). But, at the end of the day, it's up to our body's defenses to fight the virus. Depending on multiple factors, such as possible immune deficiencies, or the viral load (the amount of virus) to which we have been exopsed, our vaccine-powered enourmous capacity to fight the virus may not be enough to completly stop the virus. Mind that the likelyhood of that happening is very small. But, considering the virus is attacking thousands of people every day, it is highly likely that it does end up happening in some cases.

However, the main power of the vaccine is not stop the disease in it of itself, but to put an end to serious disease and hospitalizations. In those scenarios, the vaccine clinical trial results are even more impressive. Taking the vaccine that I took (Janssen) as an example, the vaccine had 100% eficacy in preventing death by COVID-19 in a study involving different disease variants and people from different countries 1( This means that, when compared with people who didn't get the shot, nobody who had the vaccine in the trials died of COVID-19.

These already low numbers have the tendency to go down even more as more people are vaccinated, given that heard immunity assures that the virus looses the ability to propagate because the large majority of people are immune. As such, once heard immunity has been achieved, even the people who would've been sick after taking the vaccine would be completley protected. Many conuties have eliminated entire viruses using this strategy.

The bottom line is that folks who take the vaccine are much less likely to be sick, the few that get sick are much less likely to need to be taken to a hospital and, of those, pratically no one dies of the disease. Setting aside all the noise and fear that comes from the nightly news and just looking at the raw numbers, the advantages of vaccines are immense and immediatly apparent.

Note that, in order for vaccines to have their full effect, one has to have gone through the whole vaccination schedule (this means taking all the doses of a multi-dose vaccine). Also, the body has to have had time to form its defenses after innoculation, which generally happens after 15 days. Many of the cases in which the person ends up being infected, they had not taken all the doses of the vaccine.

Am I going to get serious side effects from the vaccine?

When we are considering getting vaccinated, a thought that usually tips the scale in the wrong direction are usually the fear of having side effects. Those are the reactions from taking the vaccine that aren't the generation of immunity. All effective pharmaceuticals have them and vaccines are no exception.

The crushing majority of people will have few or no such effects, given that vaccines, because they're administred to healthy individuals, have strict safety and eficacy requirements and the results of the research into those vaccines are publicly available and are reviewed by panels of independent specialists and by international and governmental agencies.

The side effects, which are generally mild and short-lasting, are a consequence of the efectiveness of the vaccine, given that they happen as our immune system detects the vaccine contents entering our system and its generating defenses that lead to the generation of immune memory, something that is absolutley essential for lasting immunity against the COVID-19 virus. That's why symptoms such as headache and localized pain are amongst the most common side effects of a viral infection, like a flu. In the case of the vaccine however, the subside quicly because the vaccine doesn't carry the actual disease and as such we don't actually end up getting sick.

There are immensely rare cases in which some people's immune system has an exaggerated response to one of the vaccine components. This reaction is not unique to vaccines, as anyone who is allergic to shellfish can tell you. Just like in shellfish however, most people who are allergic to vaccine components already know it, either by medical diagnosis or a previous adverse reaction. To those even rarer cases of an unexpected adverse reaction, most vaccination centres are equipped to handle these situations, with nurses on site and drugs ready to be used in the unlikley case they're needed, such as in the event of anaphylatic chock or Reinke's edema.

Which all this what I mean to say is that, if there isn't any previous history of severe allergy, serious adverse reactions should not be a problem that people deciding on whether to get vaccinates should even consider.

What about long-term effects?

Everyone I heard mentioning long-term side effects, i.e. effects that only show up months or years after vaccination, are people who are not correctly informed about the specificities of these pharmaceuticals.

Historically, it is known that vaccine side effects happen up to 6 months after innoculation 2(, being practically impossible to have any side effects after that time. Clinical trials are long enough to catch any unwanted reactions that appear in that period of time and, even after the trials are done, side effects are continually monitored. As such, if there existed any long-term side effect from vaccines, those would've already came up.

The components of the vaccine only stay for a short amount of time in our bodies, as such, any related adverse effects that come up have a limited time to appear. The only long-lasting memory our body develops after taking the vaccine is the immune memory, but that's because it's generated by owr own immune system are not by any drugs that stay in our body.


All the available facts and evidence are what sustains the recommendation that is the same across practially all specialists and health authorities - anyone that can be vaccinated should do it, and as quickly as possible. It is quite normal to fear something that is new and unheard of, but the antidote to that fear is information and knowledge, something that, in the case of vaccines, has a solid foundation built throughout the years and specially in the last few months. Vaccines, like everything in life, have risks but the advantages of taking the vaccine far outweigh the risks of catching COVID-19 which, contrary to vaccines, has taken millions of lives throughout the world. I have already taken mine and you, have you done your part yet?

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