The CDC also recommends that if you've had coronavirus, you should wait 90 days before getting vaccinated. A Stanford doctor explains why:
LUZ PENA: For two weeks, 34-year-old Alejandro Galicia was in the ICU fighting for his life.
ALEJANDRO GALICIA: The doctors mentioned you're 50-50. I was in the prone position, laying on my stomach in hopes that I would get some oxygen in.
LUZ PENA: Two months ago, he was finally vaccinated.
ALEJANDRO GALICIA: I think after six hours of vaccination was when I started feeling a little weak, body aches, and I had 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.
LUZ PENA: That memory is key, and what doctors believe could be the reason why some people who've had COVID have robust side effects to the first dose of the vaccine.
PROF. DEAN WINSLOW, MD: Initial exposure sort of primes the immune system and then you have a more vigorous response after re-exposure.
LUZ PENA: Dr. Dean Winslow is a Stanford professor and advisor to the CDC. He says headaches, body aches, and a slight fever are normal side effects to the vaccine that people who've had COVID could experience sooner. What is it that the vaccine has that the body detects or remembers as COVID?
PROF. DEAN WINSLOW, MD: Presumably when people have natural infection with SARS-CoV-2 which is, of course, the virus which causes COVID-19, you get a broad probably antibody response as well as what's referred to as a cellular immune response. And there are T-cells and monocytes and macrophages are probably all activated, as well.
LUZ PENA: At UCSF, Dr. Prather is set to begin a study of 600 people after they've been fully vaccinated. Part of this study will determine if prior infection leads to a stronger immune response.
DR. ARIC PRATHER: At present, you know, people who have not been exposed or infected previously are showing efficacy of like 90, in the 90 percentile, right, 95, 93% protection. And so, you know, whether prior exposure infection actually gets you that rest of the way, like that 3% or 4% is unknown.
LUZ PENA: For Alejandro, that San Francisco resident in our story, his side effects were gone after 36 hours. Now doctors say that normal side effects are expected to dissipate after two to three days. In the newsroom, Luz Pena, ABC 7 News.