With your first dose of the Pfizer or Moderna vaccine, perhaps you felt a mildly sore arm.
But the second dose is gaining a reputation for packing a punch. About 40 to 50 percent of people experience symptoms such as fatigue, headache and even chills or a fever.
As millions of Californians await their shots, they wonder: What’s going on? We asked experts.
Q: Why do we feel side effects to vaccines?
A: “The ‘reactogenicity’ that we see — the local and systemic reactions — are short-lived and typically occur between one to three days after vaccination. They’re essentially a reflection of your body’s way of developing immunity.”
• Grace Lee, M.D., professor of pediatrics at Stanford University School of Medicine
Q: Why does the second dose cause more problems?
A: 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.”
• Grace Lee, M.D., professor of pediatrics at Stanford University School of Medicine
Q. What can I do to counter the side effects?
A. Don’t be tempted to skip your second dose. The Pfizer-BioNTech shots are spaced 21 days apart; the Moderna shots are 28 days apart. While the first dose provides some protection around 12 days, you won’t be fully protected until two weeks after your second dose.
If possible, schedule the second dose when you can get some extra rest. If you experience intense side effects, it’s safe to take acetaminophen (Tylenol) or ibuprofen (Advil). Ice may help a sore arm. Serious allergic reactions are very rare.
Q: Are there age differences in the response?
A: Older adults tend to have a milder response than younger people because “their immune systems are not responding as vigorously as a young person’s, but they still get 95 percent protection from the virus.”
• William Schaffner, M.D., professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine
Q: Why doesn’t everyone experience them?
A: Aside from age, experts don’t know why some people have more intense reactions than others do.
“Everyone responds differently. There really is a range of side effects or reactions and not everybody will have a lot. And that’s OK because we have data from the trials that even in those who don’t have side effects, there is great efficacy.”
• Grace Lee, M.D., professor of pediatrics at Stanford University School of Medicine
Q: I felt OK after the second dose. Does that mean I’m not protected?
A: Think back on your education. You might have fond memories of the fun of primary school. High school or college was harder. They both matter.
“The primary vaccination taught the easy stuff to the immune system, like in primary school. Sixth grade fractions and all of that – those are pretty easy, but important.
Does that mean that the primary school education wasn’t really an education, because it didn’t really cause you pain? No.
And then it had a secondary school or college education, which is a bit tough. It got into calculus and physics. But still, the immune system went through it, and that was really great.
In the end, you had the full education. That’s exactly how vaccines work.”
• Bali Pulendran, M.D., professor of microbiology and immunology at Stanford University School of Medicine
Q: Why don’t other vaccines cause these problems?
A: Some do.
“It happens with many, many other vaccines. With the Shingrix shingles vaccine, people sometimes say: ‘Oh my goodness, that second shot. I had a fever for a day.’
“This is not something that is unique to the mRNA vaccines. This is a general feature of any vaccine.”
• Bali Pulendran, M.D., professor of microbiology and immunology at Stanford University School of Medicine
Q: If I’m exposed to the virus after vaccination, will I also feel these symptoms?
A: Unlikely.
“It is going to depend on how much virus gets into your system. If you’re exposed, it’ll get into your nasal passages — and hopefully, it will stay there. This is what we call ‘mucosal immunity.’ It is a local immune response.
“It’s not like that second dose of the vaccine, which is getting injected into your whole system, and so the lymph nodes respond. That’s a full-body response.”
• Aruna Subramanian, M.D., clinical professor of medicine – infectious diseases at Stanford Healthcare
Q: What are “vaccine failures”?
A: “We’re learning that both of these vaccines look like they work very well in preventing serious disease and hospitalization. But in clinical trials, some people have developed symptomatic disease. And that is a sign that the vaccine isn’t working quite as well as we want it. We’d love for it to prevent all disease.”
• Annie Luetkemeyer, M.D., professor of infectious diseases at UC San Francisco
Q: Is there a chance that I’ll get infected after being vaccinated, but experience no symptoms?
A: “My guess is what we’re going to find out about asymptomatic infection — if and when it occurs after vaccination – is that people have just a little bit of virus around.
“It’s probably not going to be sufficient to really set up a robust infection or inflammatory response. I suspect we’re not going to see ‘long haul’ complications. Whether or not it’s infectious to others? That remains to be seen.”
• Annie Luetkemeyer, M.D., professor of infectious diseases at UCSF
Q: After the second dose, how long do the vaccines provide protection?
A: We don’t yet know.
“The data are incomplete because we haven’t had these vaccines for very long. But the good news is — in the time that we have had them and been able to study them — we haven’t really seen a significant waning of levels of immunity.
“Right now it looks like the ‘half life’ of the antibodies after vaccination are at least a year, and probably longer than that.”
• Joel Ernst, M.D., professor of medicine and chief of the Division of Experimental Medicine at UCSF
Q: Is it likely we’ll need ongoing “booster shots”?
A: If needed, vaccine “boosters” could help in two ways. They can recharge the immune response against the original virus. They can also help fend off a new viral variant that could render existing vaccines ineffective. But it’s not known if they’re necessary.
“What we don’t know is exactly what level of antibodies is sufficient to protect us. So there’s a lot of monitoring right now, and an increasing amount of viral sequencing to know what variants are arising.”
• Joel Ernst, M.D., professor of medicine and chief of the Division of Experimental Medicine at UCSF
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