Anonymous
Post 01/13/2026 09:24     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
Anonymous wrote:OP, there are two types of immune responses: innate and adaptive.

Innate immune responses are generalized, not specific to the antigen. They unfold the same way every time. They also happen immediately. Fever is an innate response.

The adaptive immune system is antigen-specific. Adaptive responses include making the specific antibodies needed to neutralize this antigen, and developing the specific memory cells that help your body recognize and defend against that pathogen in the future. Because the adaptive immune system is quite complex, and involves some degree of “learning” it takes quite a bit longer than the innate responses.


This is OP and this is what I was looking for. I had no idea. Thank you!
Anonymous
Post 01/13/2026 09:12     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
Anonymous wrote:OP, there are two types of immune responses: innate and adaptive.

Innate immune responses are generalized, not specific to the antigen. They unfold the same way every time. They also happen immediately. Fever is an innate response.

The adaptive immune system is antigen-specific. Adaptive responses include making the specific antibodies needed to neutralize this antigen, and developing the specific memory cells that help your body recognize and defend against that pathogen in the future. Because the adaptive immune system is quite complex, and involves some degree of “learning” it takes quite a bit longer than the innate responses.


+1. To think of it more simply, imagine someone breaks into your compound. First step is to send in the rapid response guys to take him out. They might be bumbling and slow or wildly overshoot while they figure out the right response to take out the threat.

After you’ve taken out the threat, then you clean up the mess, survey the perimeter, and develop a plan to target similar threats more precisely next time. The plan is what takes 2 weeks.
Anonymous
Post 01/13/2026 08:39     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
OP, there are two types of immune responses: innate and adaptive.

Innate immune responses are generalized, not specific to the antigen. They unfold the same way every time. They also happen immediately. Fever is an innate response.

The adaptive immune system is antigen-specific. Adaptive responses include making the specific antibodies needed to neutralize this antigen, and developing the specific memory cells that help your body recognize and defend against that pathogen in the future. Because the adaptive immune system is quite complex, and involves some degree of “learning” it takes quite a bit longer than the innate responses.
Anonymous
Post 01/13/2026 08:16     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
My DH usually gets two days of fever after the flu shot. I typically don’t get a fever at all.
Anonymous
Post 01/13/2026 07:42     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
Anonymous wrote:I’ve never gotten a fever after a flu shot.


PP here, and I haven’t either - only from the Covid shot.
Anonymous
Post 01/13/2026 07:41     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
Anonymous wrote:I’ve never gotten a fever after a flu shot.


It's more common in the young who have and very intune immune system.
Anonymous
Post 01/13/2026 07:41     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
The fever is your immune system reacting to the vaccine. It does not last longer than a day because you are not actually ill with a virus - the flu shot does not cause the flu.

Anonymous
Post 01/13/2026 07:40     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
I’ve never gotten a fever after a flu shot.
Anonymous
Post 01/13/2026 07:38     Subject: Re:If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
Your body notes there is a foreign substance and is fighting it.

If you have a terrible immune system, this will not happen.
Anonymous
Post 01/13/2026 07:35     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
Anonymous wrote:I don’t understand the question. Day 1 of what? Maybe I’m just dense?


You get the vaccine today, you get a fever today or maybe tomorrow, but yet it's supposedly two weeks to build up the antibodies. Why is the fever soon after the shot and not longer and/or later?
Anonymous
Post 01/13/2026 07:33     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
Anonymous wrote:Your immune system is ramping up.


Why does the fever stop then? If you have the actual illnesses, doesn't the fever breaking mean you are getting better? Do you still build up antibodies for another 2 weeks after the fever breaks?

It's not the fever I'm trying to figure out but the timing as compared to the two weeks for the vaccines to be fully effective.
Anonymous
Post 01/13/2026 07:33     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
I don’t understand the question. Day 1 of what? Maybe I’m just dense?
Anonymous
Post 01/12/2026 22:03     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
Your immune system is ramping up.
Anonymous
Post 01/12/2026 21:50     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
Your immune system is doing its job.
Anonymous
Post 01/12/2026 20:45     Subject: If Flu and Covid vaccines take 2 weeks to work, why is fever on day 1?
If Flu and Covid vaccines take 2 weeks to be fully effective, why is fever, if any, on day 1? What is the fever doing, and why doesn't it last longer?