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What side effects should I expect from COVID-19 vaccines?

After either shot, one can experience fever, chills, swelling of the lymph nodes, and headache. It is often worse after the second dose due to your immune system being “primed” for activation.


How long do side effects last?

Generally 24-48 hours. I have mostly seen 24 hours.



Since COVID-19 vaccines come in two doses, will I experience the same side effects after each dose?

Often the second dose has a more vigorous response.


Do the side effects differ from one manufacturer’s vaccine to another?

Side effects are similar after the Pfizer and Moderna mRNA vaccines but could differ with other types of vaccines.



Why do these vaccines seem to have more noticeable side effects than the flu vaccine? How do they compare to other vaccines’ side effects—like the shingles vaccine, for example?

All vaccines can cause these same effects as all vaccines to trigger the immune system. In my practice, I have patients who do get fevers and chills from the flu vaccine as well. I will admit that the amount of people who have these side effects is somewhat higher than with other vaccines. I don’t know the reason for this, but I will assume it is because the mRNA technology not only forms antibodies like other vaccines but also stimulates B and T immune cells leading to a more vigorous response.


Can side effects be more pronounced in people who are at higher risk of severe COVID-19 disease—i.e., people who are older, have comorbidities, etc.?

Unlikely, actually, because this population will likely have a weaker immune response. It is actually the immune response that causes symptoms, (this is also the case for diseases themselves like influenza. It’s not the virus that makes a person feel bad, it is the body’s immune reaction that does this.) Therefore a less effective immune system will often have a less vigorous response.



Do children experience side effects from COVID-19 vaccines?

Currently, the vaccine is only approved for age 16 and up. Studies are being done on the 12-16-year-old population.




How do I know if what I am experiencing is normal or if I should alert my doctor or health care provider?

Any time you have what you consider significant issues, you should alert a health care provider. You can expect a fever up to 103, but if it lasts more than 24 hours, you should seek attention The Centers for Disease Control and Prevention advises that you should contact your doctor or health care provider if the redness or tenderness where you got the shot increases after 24 hours, or if your side effects are worrying you or do not seem to be going away after a few days.



Is it possible for me to contract COVID-19 between doses?

Absolutely. The vaccine does not prevent you from contracting COVID at all. It is important to continue to wear a mask, practice social distancing, and wash your hands. Instead, it causes your immune system to gear up to fight it quickly, thus keeping it from replicating and causing severe disease. The first dose will not provide complete protection, and it will take about seven

days after your second dose before you will achieve a full protective level of immunity that develops in about 95% of vaccine recipients. Even once you have received both doses of the COVID-19 vaccine, it will still be important to continue practicing public health mitigation strategies like masks and distancing due to the fact that the nature of the vaccine is that it will help your body fight the infection much more effectively, but it WILL NOT prevent you from contracting or spreading the virus.


How will I know whether what I am experiencing are side effects or possible COVID-19 infection?

If the effects don’t go away in 24-48 hours, seek medical attention.

Second dose of COVID vaccine from Pfizer or Moderna is needed, but timing doesn’t have to be exact.


Chief Medical Officer
Davis Health System

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People who get one of the COVID-19 vaccines will be expected to get their second shot 21 or 28 days after the first one, depending on the manufacturer. But what happens if someone misses that deadline? At this time distribution of the vaccine can be very uncertain, and at Davis Health System we want to answer your questions about the timing of your second dosage.

How long will you have to wait for a second dose?

If you received Pfizer, it will be revaccinated at three weeks, and the Moderna vaccine is four weeks. Vaccines are designed to create immunological memory, which gives our immune system the ability to recognize and fend off invading foes even if we have not encountered them before. Two-shot vaccinations aim for maximum benefit: the first dose primes immunological memory, and the second dose solidifies it.


How protective is the first vaccine dose?

On its own, the first dose will not be as effective at decreasing the severity of the disease if contracted. The vaccine is developed to be a two-part vaccine.


Will delaying the second dose make the vaccine less effective overall?

The vaccine was studied to be given at specific intervals. Other intervals were not studied. Theoretically, it can be assumed that you can effectively get the second dose up to 6 weeks after the first, but it cannot be said for certain what would happen to immunity in this case as it was not studied.


Do the two doses need to be from the same manufacturer?

Yes, unless it is completely impossible to do so. Again, using the two manufacturers interchangeably was not studied, and therefore there is no data as to the efficacy of this practice.


What are the drawbacks of delaying the second dose?

A drawback is a lack of knowledge as to the efficacy the longer you wait. There are cases up to 6 weeks, that seem to continue to have immunity that is why the CDC is accepting up to 6 weeks only in extenuating circumstances.