Medical experts say having side effects is a normal sign that your body is producing an immune response, and explain why this could be happening.
RELATED: Here's why some are experiencing side effects after 2nd Pfizer or Moderna vaccine shot
For two weeks, 34-year-old Alejandro Galicia Diaz was in the ICU fighting for his life.
"The doctor mentioned, 'you're 50/50.' I was in the prone position laying on my stomach in hopes that I would get some oxygen in," said Galicia, describing his time in the hospital.
Two months ago he was finally vaccinated.
Luz: "When did you realize that you were going through the first wave of side effects of the vaccine?"
Alejandro Galicia: "I think after 6 hours of vaccination. That is when I started feeling a little weak, aches and a slight fever."
Luz Pena: "Did it take you back to what you went through with COVID?"
Alejandro Galicia: "It did. The body aches were a little rough and one of the first symptoms that I developed when I had COVID was body aches. I couldn't get up, I couldn't move. So mentally it brought me back to that point."
That memory is key, and is what some doctors believe could be the reason why some people who've had COVID-19, have robust side effects to the first dose of the vaccine.
VIDEO: Pfizer CEO says 3rd COVID vaccine dose likely needed within 6 to 12 months
"Initial exposure sort of primes the immune system, and then you have a more vigorous response after re-exposure. That is one reason actually that the messenger RNA Vaccines utilize this two dose regiment," said Dr. Dean Winslow, Stanford Professor of medicine and Infectious Disease Specialist.
Dr. Dean Winslow says headaches, body aches and a slight fever are normal side effects to the vaccine that people who've had COVID could experience.
Luz Pena: "What is it that the vaccine has that the body detects or remembers as COVID?"
Dr. Dean Winslow: "Presumably when people have natural infection with SARS-CoV2. Which is the virus that causes COVID-19. You get a broad probably antibody response as well as what is referred to as cellular immune response... then when you are re-exposed you have a very rigorous reaction."
At UCSF, Aric Prather will begin studying 600 people after they've been fully vaccinated.
Luz Pena: "What are you hoping to find?"
Aric Prather: "The BOOST study, building optimal antibody study. We are trying to understand kind of behavioral and psychological and immunologic predictors of how well people respond to the COVID -19 vaccines."
Part of his study will determine if prior infection leads to a stronger immune response.
"At present people who have not been exposed or infected previously are showing efficacy on the 90 percentile. 95 - 93% protection. Whether prior exposure or infection gets you that 3 or 4% rest of the way is unknown," said Prather.
According to Dr. Winslow normal vaccine side effects could last anywhere between two to three days.
VACCINE TRACKER: Here's how CA is doing, when you can get a coronavirus vaccine
The CDC recommends to get vaccinated 90 days after being cleared from the infection. But, why 90 days?
"The guidance is sort of unclear in many cases. Certainty, if you received the monoclonal antibody treatment with your initial infection, there are good reasons. Theoretically that you should wait 90 days because it takes that long for those antibodies to dissipate and it will potentially block the immune response to the vaccine," explained Dr. Winslow.
Having trouble loading the tracker above? Click here to open it in a new window.
RELATED STORIES & VIDEOS:
- Map shows which counties can, can't reopen under reopening tiers
- Cheat sheet: What you can and can't do after being fully vaccinated
- How to register for a COVID-19 vaccine in every Bay Area county
- Map shows everywhere you can get a COVID-19 test in the Bay Area
- Interactive map shows what's closed and what's reopening in the San Francisco Bay Area
- Data tracker: Coronavirus cases, deaths, hospitalizations in every Bay Area county
- Third stimulus check calculator: See how much you could get
- COVID-19 Diaries: Personal stories of Bay Area residents during pandemic
- Get the latest updates on California EDD, stimulus checks, unemployment benefits
- Coronavirus origin: Where did COVID-19 come from?
- What is a COVID-19 genetic, antigen and antibody test?
- What does COVID-19 do to your body and why does it spread so easily?
- Coronavirus Timeline: Tracking major moments of COVID-19 pandemic in San Francisco Bay Area
- Coronavirus Doctor's Note: Dr. Alok Patel gives his insight into COVID-19 pandemic