What's going on with the second COVID shot?
April 5, 2021 8:38 AM   Subscribe

I'm due to get the 2nd Covid shot soon (TEAM PHIZER) and have been warned that I should count on feeling sick for a day or two afterwards. Why is that? I've read various explanations, but it's not making logical sense to me, can you break it down for me, real simple like?

Is the vaccine essentially like a fake COVID virus? So the first dose teaches the body what to look for and second dose is the "fake" virus, so the body reacts with "OH that's the mofos we were warned about, lets go mess them up" ?

If so, why is the second dose needed? Didn't the body learn what to look for during the first dose? Is the second one like a test?

And why do some vaccines require two shots and at least one only require a single shot?
posted by Brandon Blatcher to Health & Fitness (13 answers total) 5 users marked this as a favorite
 
Here's a nice explainer aimed at patients from the American Medical Association. Basically, the first dose stimulates a weak response to the virus, and the second dose stimulates a stronger response, increasing your immunity.
posted by hydropsyche at 8:54 AM on April 5 [6 favorites]


More entertaining than the AMA, I refer you to the tiktok legend, https://twitter.com/hotvickkrishna/status/1377731963101061123
posted by allegedly at 8:55 AM on April 5 [28 favorites]


Here's more info from New Scientist (aimed at a UK audience though still largely relevant).

Basically you MAY feel sick afterwards because your immune system is learning how to fight the virus. There's a chance you might not get any symptoms, or only have very minimal symptoms. Please keep in mind that a lack of symptoms doesn't mean that the shot "hasn't worked", it just means that's your body's reaction.

Your analogy is correct; some vaccines require two doses to "fine tune" the immune response. Others, which work differently, only need one shot to be broadly effective.
posted by fight or flight at 9:00 AM on April 5


A somewhat flippant, but I think still correct, answer to your last question as to why Moderna and Pfizer have two doses while Johnson & Johnson has a single dose is that that's what they did in the trials to get FDA approval. (Currently. I believe all of the major currently approved vaccines are under going trials of boosters of either the identical vaccine or slightly modified ones to better target the new variants. So that means after these trials, new vaccinations/booster schedules might be different than what we have now. Like, J&J at least has a trial underway looking at different multi-dose schedules.)
posted by skynxnex at 9:13 AM on April 5 [4 favorites]


To put it plainly, the different vaccines have different ways of stimulating your immune system. FDA basically want a vaccine to induce enough antibodies to count as "effective" while needing speed, speed, and more speed. Moderna and Pfizer basically said "we can do this in two dose now, we think we should use this, instead of trying to tweak it some more for a single dose solution" while J&J said "we got a solution that works in one dose". And FDA says "I choose you, you, and you!" :)

The specific mechanisms aren't really discussed, and I think we'll need some serious virology and microbiology and immunology to understand it anyway. :)
posted by kschang at 9:24 AM on April 5 [1 favorite]


April 1st I got my 2nd dose of Pfizer. I felt a bit tired and the vaccination spot hurt a little when on the 1st it didn't hurt at all and I wasn't tired. Then when I was taking a shower I noticed big lump near my arm pit on the side where I got the shot. Googled and that's a lymph node and of course it could swell. By now no swelling and arm doesn't hurt. This is just one data point for your research.
posted by forthright at 9:58 AM on April 5


I got Pfizer dose #2 today and I'm feeling OK so far. I've had some allergy symptoms lately so I was already sniffling and sort of feeling blah. The first shot was a breeze. My mom had a rough ride with the first shot and said she's feeling similarly today after getting her second dose yesterday. My wife only felt bad after the second dose. YI*MV, I guess.

* immune system
posted by emelenjr at 10:09 AM on April 5


So, first of all the reason you have a second shot is because your immune system is basically going to take the virus even more seriously if it sees it twice. When your immune system sees something that it already recognizes, it's like, "Oh I know this asshole! This spike protein (and the virus attached to it) is going DOWN!"

As for why that translates to you feeling like crap: a lot of symptoms of viral infections are caused by your immune system. Fever: mostly caused by your body trying to make it unpleasant for an infectious agent to hang out and reproduce in your body. Sneezing, snot, coughing: mostly your body trying to flush gross stuff out of your body. Aches: white blood cells, etc., causing inflammation that makes you feel like garbage. Your own immune system can really do a number on you.
posted by mskyle at 10:24 AM on April 5 [4 favorites]


The vaccine basically mimics the virus, yes--it tries to present apiece of the virus to your body as if it were a sign of an infectious pathogen. You get the vaccine side effects as your immune system leaps into gear to fight it.

As to why you might need two, your body has a bunch of feedback loops to respond to infection--and avoid overresponding. The initial response includes making some antibodies and also memory cells. These make the next response faster, which is the whole point of a vaccine. But a second injection might trigger the body to produce many more times of these things. It's not really a test, so much as "same thing as the first time, only faster and better."

As to why some vaccines need one and others need two, it's not 100% clear we're doing this optimally. Maybe you don't. But the vaccines are different. The Pfizer/Moderna ones produce the virus protein (the antigen) by getting mRNA into you, which your cells use to create the protein itself. I'm not up on the details, but I doubt the mRNA lasts long, so it'd be reasonable for researchers to worry that you need two hits to get the full response. The J&J gives the antigens directly.

But that's the sort of thing you test, and based on the timelines everyone just tested their best, most likely to succeed, hypothesis. So that's what got approved and how it's being distributed.
posted by mark k at 10:27 AM on April 5 [3 favorites]


The J&J gives the antigens directly.

This is not quite right. The J&J vaccine is a harmless virus that has been modified to produce the spike protein from the coronavirus. So the harmless virus makes a whole bunch of copies of itself in your body, giving your immune system plenty of targets, and then your immune system clears the harmless infection afterward. This is the same basic strategy as the AstraZeneca vaccine and some of the Chinese and Russian vaccines.

The Novavax vaccine, which has not yet been approved but is doing well in trials, does take the direct approach of (basically) just injecting a bunch of the bare spike protein and not relying on replication within the body.
posted by jedicus at 11:00 AM on April 5 [2 favorites]


So the harmless virus makes a whole bunch of copies of itself in your body, giving your immune system plenty of targets, and then your immune system clears the harmless infection afterward.

Still not quite right. The J&J vaccine is non-replicating, non-replicating vector vaccines are unable to make new viral particles; they only produce the vaccine antigen.
posted by doctord at 12:40 PM on April 5 [8 favorites]


My 2nd Pfizer shot was fine until about 24 hours after, then I felt slightly fluish. I slept like a log that night and woke up feeling great. I had several days of slight achyness after the first shot.

The point is that reactions vary widely, and this is completely expected and normal.
posted by lhauser at 12:41 PM on April 5 [1 favorite]


Yeah, I messed up the J&J description badly, thanks for correcting that. I was trying to cut my post short but should have left it at "they're different" rather than writing incorrect words.
posted by mark k at 4:45 PM on April 5


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