Vaccinated? Here’s what to expect


So you have received a vaccine in hopes of preventing moderate or severe infection with Sars-Cov-2. You may have noticed friends and family who have received different vaccines.  Currently four vaccine manufacturers are prevalent, and the availability of those vaccines is dependent upon authorization by the U.S. Food and Drug Administration as well as vaccine supply.

As a quick reminder, a vaccine’s purpose is to trigger an immune response to protect against an infectious disease. Currently two differing delivery systems exist to trigger our immune response with COVID vaccination.

Johnson and Johnson (J&J) (currently on pause) and AstraZeneca render an adenovirus harmless to mimic a COVID-19 infection and have the body create antibodies to future Sars-Cov-2 exposure. Pfizer and Moderna rely on mRNA technology to introduce cellular code that makes the body create antibodies, but no virus is required. For further explanation, see the previous article identified below.

All of the current vaccines instruct some of our own cells to make the novel spike protein found in Sars-Cov-2. As we mount an immune response to this protein, side effects and symptoms may occur. The most common are as follows.

Local side effects

  • Pain at injection site
  • Redness at site
  • Swelling at site

Systemic side effects

  • Fatigue
  • Headache
  • Muscle pains
  • Joint pain
  • Fever
  • Nausea
  • Allergic reaction

Possible severe reactions

Of note is that severe allergic reactions can include anaphylaxis. Anaphylaxis is a kind of severe allergic reaction that occurs rapidly, sometimes minutes after an individual is exposed to a medication or allergen. Symptoms may include airway irritation or low blood pressure, which may lead to lightheadedness, unconsciousness or difficulty breathing.

This is why one waits 15 to 30 minutes after vaccination looking for any severe symptoms. If this were to happen, a health care provider would intervene. Fortunately, interventions have rarely been needed as very few severe reactions have occurred during the 126 million vaccinations from 12/14/20 to 3/22/2021.

In a recent study of vaccinated employees at Mass General Hospital, only 2% had an allergic reaction to either Pfizer or Moderna.  Even more impressive was that only a tiny percentage, .025%, had anaphylaxis. Of those with anaphylaxis, 63% had a prior history of anaphylaxis or allergic reaction history, so we know to be cautious when an individual has a history of either.

Most vaccine-related reactions have been local side effects, with 80% of people reporting these side effects after either their first or second dose of Moderna or Pfizer products. Fewer side effects are seen with the J&J-based vaccine as of this date. Of note, all of the systemic side effects are more frequent in young adults with the exception of nausea, which was seen in young and old.

Many of the systemic symptoms are more common after the second dose of the two mRNA-based vaccines. The body’s immune response is primed to respond to the spike proteins once inside our cells regardless of the kind of vaccine administered. If you don’t have side effects, don’t worry. It doesn’t mean you haven’t mounted an immune response or that the vaccine isn’t working.

It is important to report side effects after vaccination so the incidence can be tracked and adjustments made as needed. The general public is encouraged to report side effects experienced after vaccination. Methods of reporting include utilizing the V-safe smartphone app or the Vaccine Adverse Event Reporting System (see information below).

Effectiveness timelines

The timeline to achieve the optimum effectiveness of the vaccines differs for each and is as follows. (For any of the vaccines, the effectiveness increases over time from initial dosing.)

  • Pfizer/BioNTEch (for use in age 16 and older)

One week after 2nd dose => 95% effective preventing symptomatic COVID-19

Two weeks after 2nd dose => 100% effective against hospitalization and death from COVID

  • Moderna (for use in age 18 and older)

Two weeks after 2nd dose => 94.1% effective preventing symptomatic COVID-19 infection

Two weeks after 2nd dose => 89% effective against hospitalization and 100% against death from COVID

  • J&J (for use in age 18 and older)

Two weeks after 1st dose => 66.9% effective in preventing moderate to severe COVID-19 infection

Two weeks after 1st dose => 76.7% severe to critical infection

Four weeks after => 85.4% severe to critical COVID-19 infection

Four weeks after =>100% effective against hospitalization and death from COVID

The Astra Zeneca vaccine that may be released in the near future is reported to be 79% effective against symptomatic COVID-19 after two doses four weeks apart and 100% effective against hospitalization or death in same time frame. There is some controversy regarding the data utilized to derive these numbers, and thus we must wait for that to be resolved before use is authorized.

All manufacturers seek to improve the effectiveness of their vaccines over time. Currently the best vaccine is the one that is offered to you. As with many vaccinations, the dosing schedules and amounts may change as more study data is available.

Booster vaccines may be required in the future for coronavirus variants or to continue effectiveness. We don’t know how long vaccine-based immunity will last, but hopefully it will be a long time.

See prior article for further explanation: How does the Johnson & Johnson vaccine compare to other coronavirus vaccines? 4 questions answered

For reporting side effects, visit V-safe After Vaccination Health Checker or

Sean J. Ennevor, M.D. graduated with a B.A.S. in biology and economics from Stanford University, and as a Dean’s Scholar from UCLA School of Medicine where he received his MD. He completed his medical residency and fellowship in anesthesiology at Yale University, where he was chief resident and on staff. He practiced medicine in the Twin Cities for over 14 years, and presently serves as an advisor and investor for medical technology companies throughout the country.