As anyone who received a COVID vaccine sourced from Pfizer or Moderna can attest, the flu-like symptoms that can occur post-injection typically manifest after the second dose. More often than not, the only symptoms people experience after the first injection are limited to minor swelling and pain around the point of injection.

At first glance, it’s a bit curious why the first vaccine dose yields minor symptoms while the second dose can sometimes leave people completely knocked out for a full 24-hours. In fact, one volunteer who participated in Pfizer’s clinical trial likened the side effects from the second dose to a “severe hangover” that caused headache, fever, chills, and severe fatigue.

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Naturally, there’s a perfectly cogent and scientific explanation as to why people tend to react more strongly to the second dose than the first.

As Dr. William B. Greenough III of Johns Hopkins explains, the first vaccine dose starts revving up the body’s immune system. And seeing as how Pfizer and Moderna’s COVID vaccines are built around Messenger RNA (mRNA) technology, the first dose essentially informs the body how to prepare for and defend against the coronavirus.

That being the case, Greenough notes that by the time the second dose is administered, the immune system is already primed to react and brings out “the big guns.”

Stanford Professor Dr. Grace Lee adds:

The first shot teaches your immune cells to recognize the virus; it’s revving up. With the second shot, there are more immune cells ready and waiting to launch a major defense. The muscle ache and fever come from inflammation; your immune cells are sending out an alarm in the form of chemicals called cytokines.

“Your immune system is ‘primed’ with dose one. You’re getting ‘boosted’ with dose two. That reflects your body’s quick response. … Your body is seeing it for the second time and remembering it, and is developing the powerful immune response that it needs to respond to infection.”

Notably, not everyone experiences side effects after the second vaccine dose. This, however, shouldn’t be taken to mean that the vaccine isn’t working. People react differently to the COVID-19 vaccine and some people are just lucky enough to not experience any severe symptoms. Suffice it to say, if you don’t experience any side effects, you can rest easy knowing that your body is still developing a strong immune response to the virus.

If you have a vaccination appointment coming up, the CDC notes that people should avoid taking pain medications like Tylenol or Motrin ahead of time insofar that doing so could impact the efficacy of the vaccine. And while not an official directive from the CDC, some health professionals are cautioning people to avoid drinking alcohol before their injections.

“Most of the available data on how alcohol impacts the immune system and vaccine responses suggest that, in general, people should avoid binge drinking and heavy drinking around the time of the vaccination,” Professor Christopher Thompson of Loyola University Maryland told Healthline recently. “Ideally, this would be avoided for at least a week before the first dose and one month after the second dose.”

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A life long Mac user and Apple enthusiast, Yoni Heisler has been writing about Apple and the tech industry at large for over 6 years. His writing has appeared in Edible Apple, Network World, MacLife, Macworld UK, and most recently, TUAW. When not writing about and analyzing the latest happenings with Apple, Yoni enjoys catching Improv shows in Chicago, playing soccer, and cultivating new TV show addictions, the most recent examples being The Walking Dead and Broad City.