Vaccine Overview & Safety

The FDA has autho­rized the Pfiz­er and Mod­er­na COVID-19 vac­cines for emer­gency use in the Unit­ed States. Vac­cines are cru­cial in sav­ing lives and erad­i­cat­ing dis­eases caused by virus­es or bac­te­ria. Due to the wide-spread COVID-19 pan­dem­ic, the devel­op­ment of a vac­cine to pro­tect against the virus has pro­gressed on extreme­ly quick­ly. Here our infec­tious dis­ease experts share details on how the vac­cine has been devel­oped safe­ly and how the vac­cine works in reduc­ing your risk of con­tract­ing COVID-19.

mRNA Vac­cine Science

Both the Pfiz­er and Mod­er­na COVID-19 vac­cines use mes­sen­ger RNA (mRNA) to pro­mote an immune response in your body to SARS-CoV­‑2, the virus that caus­es COVID-19. Our bod­ies use pro­teins each day to keep us healthy. mRNA pro­vides your cells with instruc­tions on which pro­teins to pro­duce, in this case anti­bod­ies to fight the COVID-19 virus. Once the instruc­tions are rec­og­nized and our body begins pro­duc­ing anti­bod­ies, the cell breaks down and gets rid of the mRNA. Dur­ing this process, mRNA nev­er enters the nucle­us of our cells, where our DNA is stored.

mRNA vac­cine ingre­di­ents include mRNA and lipids to ensure safe deliv­ery of the mRNA to your cells. Nei­ther the COVID-19 Pfiz­er or Mod­er­na vac­cines con­tain adju­vants (alu­minum salts) or preser­v­a­tives (eth­yl­mer­cury). They also do not include weak­ened or inac­tive ver­sions of the virus some­times found in tra­di­tion­al vac­ci­na­tions. It is impos­si­ble to con­tract COVID-19 from these vaccines.

Vac­cine Safety

Like any vac­cine, both Pfiz­er and Moderna’s COVID-19 vac­cines have gone through clin­i­cal tri­als to ensure high-lev­­els of safe­ty. Thou­sands of par­tic­i­pants received the vac­cine and side effects, if any, were record­ed. All four phas­es with­in the tri­al must be passed before mov­ing on to the FDA approval2 process.

  • Phase I: Uti­lizes a small group of vol­un­teers to test the gen­er­al safe­ty of the vac­cine. Adjust­ments are made to ensure the high­est lev­el of safe­ty before mov­ing on to the next phase.
  • Phase II: Incor­po­rates a slight­ly larg­er group of vol­un­teers from a vari­ety of demo­graph­ics. Par­tic­i­pants will receive vary­ing dos­es of the vac­cine to help researchers deter­mine dosage. Short-term side effects and immune respons­es are recorded.
  • Phase III: Vac­cine is now typ­i­cal­ly admin­is­tered to thou­sands of par­tic­i­pants to com­pare immune respons­es between those who were vac­ci­nat­ed and those who received the placebo.
  • Phase IIII: This final stage occurs after FDA approval. The vac­cine is now avail­able for the gen­er­al pub­lic to receive. Safe­ty and effi­ca­cy are observed on a larg­er scale.

For COVID-19, Emer­gency Use Autho­riza­tion was passed since FDA approval can require ample time for licensure. 

Under­stand­ing FDA Emer­gency Use Authorization

Before any vac­cine is per­mit­ted for use in the Unit­ed States, it must be reviewed and approved by the Food and Drug Admin­is­tra­tion (FDA). This process can some­times take years, but in instances such as a pan­dem­ic, we don’t have years to wait. Emer­gency Use Autho­riza­tion (EUA) is a tool the FDA uses to help make impor­tant med­ical prod­ucts, such as vac­cines, avail­able more quick­ly dur­ing a pub­lic health emergency. 

In order for the COVID-19 vac­cines to be approved for use, all clin­i­cal tri­als must show they are safe and effec­tive. The process fol­lows an open and trans­par­ent review that includes input from both inde­pen­dent sci­en­tif­ic and pub­lic health experts as well as FDA sci­en­tists. The known and poten­tial ben­e­fits of the vac­cine have to out­weigh the known and poten­tial risks of the vac­cine in order for it to be grant­ed EUA sta­tus. Even with quick approval, the FDA still ensures that the vac­cine has met their rig­or­ous sci­en­tif­ic stan­dards for safe­ty, effec­tive­ness, and man­u­fac­tur­ing quality.

Pfiz­er and Mod­er­na Vaccines

The Food and Drug Admin­is­tra­tion (FDA) has grant­ed two com­pa­nies, Pfiz­er and Mod­er­na, Emer­gency Use Autho­riza­tion for their COVID-19 vac­cines. Both vac­cines are now being dis­trib­uted to the pub­lic with­in the Unit­ed States.

They are both mRNA vac­cines which work by teach­ing our cells how to make pro­teins that prompt an immune response with­out using a live virus. Once the immune response is trig­gered, our body then makes anti­bod­ies to fight the infec­tion if the real COVID-19 virus enters the body in the future. 

The Pfiz­er and Mod­er­na vac­cines are very sim­i­lar when it comes to how safe and effec­tive they are, how­ev­er, there are a few differences: 


Avail­able only at our Lisle clinic


Avail­able only at our Glen Ellyn and Tin­ley Park clinics

Who can receive the vaccine?

Peo­ple 12 years and older

Peo­ple 5 — 11 years old receive a Pedi­atric dose

Peo­ple 18 years and older


To be max­i­mal­ly effec­tive, the Pfiz­er vac­cine needs two dos­es giv­en 21 days apart. You must receive two dos­es of the same vaccine. 

To be max­i­mal­ly effec­tive, the Mod­er­na vac­cine needs two dos­es giv­en 28 days apart. You must receive two dos­es of the same vaccine.

Ship­ping, Stor­age & Tem­per­a­ture Requirements 

The vac­cine needs to be shipped with dry ice and main­tain a tem­per­a­ture of less than ‑94 degrees Fahren­heit. The vials can be placed in a refrig­er­a­tor for up to five days and need to be used with­in six hours of thawing.

The vac­cine can be shipped and main­tained at stan­dard freez­er tem­per­a­tures of ‑4 degrees Fahren­heit. The Mod­er­na vials can be stored in a refrig­er­a­tor for up to 30 days and must be used with­in 12 hours once thawed.

Vac­cine Preparation

After thaw­ing, the vac­cine has to be dilut­ed with saline (salt­wa­ter) before it can be inject­ed. This is a com­mon prac­tice for vaccines.

This vac­cine does not require any dilu­tion and comes ready to be administered.

Vac­cine Series

Both Pfiz­er-BioN­Tech and Mod­er­na COVID-19 vac­cines are con­sid­ered to be vac­cine series, indi­cat­ing the need for two dos­es. Researchers stress the impor­tance of sched­ul­ing the sec­ond dose in order to have the high­est pro­tec­tion against COVID-19. Pfiz­er-BioN­Tech rec­om­mends hav­ing two dos­es 21 days apart and Mod­er­na rec­om­mends two dos­es 28 days apart. There is lim­it­ed research on the effec­tive­ness of patients receiv­ing one dose of the vac­cine at this time.

Between dos­es you can still con­tract COVID-19. Data has shown that vac­cine par­tic­i­pants had only 52 per­cent effi­ca­cy against COVID-19 after the first dose. 

After the sec­ond dose, your risk of seri­ous com­pli­ca­tions from COVID-19 infec­tion are dras­ti­cal­ly reduced. Data has shown that it can take one to two full weeks for your body to reach full efficacy. 

To pro­tect oth­ers, experts high­ly rec­om­mend con­tin­u­ing social dis­tanc­ing efforts such as wear­ing masks after vac­ci­na­tion. Receiv­ing the vac­cine helps build up your immu­ni­ty to COVID-19 if you are exposed. The vac­cine can also help pro­tect you from con­tract­ing seri­ous side effects from COVID-19. 

  • Social dis­tanc­ing efforts: 
    • Wear a mask over your nose and mouth
    • Avoid group gathering
    • Stay six feet apart from others
    • Wash your hands
    • Wipe down com­mon­ly used surfaces

Side Effects

Mild side effects, such as those list­ed below, occur com­mon­ly and typ­i­cal­ly resolve with­in 24 – 48 hours of the vac­ci­na­tion admin­is­tra­tion. Com­mon side effects of the COVID-19 vac­cine include:

  • Pain or swelling in the arm that received vaccine
  • Fever
  • Chills
  • Tired­ness
  • Headache

These typ­i­cal side effects can be treat­ed with rest, hydra­tion and Tylenol or ibupro­fen. Patients should call their doc­tor if they feel that their side effects are becom­ing severe.

Aller­gic Reac­tions: Those with a known his­to­ry of aller­gic reac­tions to injectable med­ica­tions or vac­ci­na­tions should con­sult with their pri­ma­ry care provider or an aller­gist before receiv­ing the vac­cine. The CDC rec­om­mends that patients who have an aller­gic reac­tion to the first dose of the COVID-19 vac­cine should not receive the sec­ond dose. If you have ques­tions regard­ing your vac­cine options, please con­tact your pri­ma­ry care provider or the Duly Health and Care Aller­gy, Asth­ma and Immunol­o­gy department.

Onsite Safe­ty Mon­i­tor­ing: All patients will be mon­i­tored for reac­tions to the vac­cine for 15 min­utes. Those with a his­to­ry of severe aller­gic reac­tions will be mon­i­tored for up to 30 min­utes. If you expe­ri­ence a reac­tion after leav­ing your appoint­ment, seek med­ical atten­tion immediately.

Fol­low-up Check-ins: Patients who receive the vac­cine will be pro­vid­ed instruc­tions on how to report side effects. A MyD­MGHealth account will allow you to fill out check-in ques­tion­naires to report any health con­cerns fol­low­ing your vaccination.

Immuno­com­pro­mised Individuals 

The COVID-19 vac­cine may pro­vide an oppor­tu­ni­ty for those at high risk for devel­op­ing seri­ous com­pli­ca­tions from the virus to have added immunity. 

Due to lim­it­ed data, it is rec­om­mend­ed for those who are immuno­com­pro­mised to con­sult with their pri­ma­ry care physi­cian before receiv­ing the vac­cine. Your provider will help inform you about the lim­it­ed data sur­round­ing peo­ple who are immuno­com­pro­mised and vac­cine safety. 

  • Who is con­sid­ered immuno­com­pro­mised2?
    • Can­cer patients
    • Those who have had a bone mar­row transplant
    • Those who have had a sol­id organ transplant
    • Received stem cells for can­cer treatment
    • Those with genet­ic immune deficiencies
    • Those with HIV
    • Those who chron­i­cal­ly use oral or intra­venous cor­ti­cos­teroids or immunosuppressants