General COVID-19 Vaccine Information

Everyone 5 years of age and older is now eligible to get a COVID-19 vaccination. Get a COVID-19 vaccine as soon as you can. Widespread vaccination is a critical tool to help stop the pandemic.


How do the Pfizer and Moderna vaccines work?

These vaccines consist of genetic material called mRNA encased in tiny particles that shuttle it into our cells. From there, it stimulates the immune system to make antibodies that protect against the virus. These vaccines do not have any impact on our genes. The vaccine material breaks down in the body shortly after it is taken into our cells.

How does the Janssen/J&J COVID-19 vaccine work? 

The Janssen/J&J vaccine involves a complex of an adenovirus (a common virus that can cause mild respiratory illnesses) and COVID-19 spike protein. The adenovirus acts as a shuttle (or vector) to transport the COVID-19 spike protein into human cells.  Our immune system then reacts against the spike protein to develop antibodies which then protect us from COVID-19 infection.

It is important to note that the adenovirus-spike protein complex has been altered so that:

  • It is unable to replicate and cannot cause infection;
  • It does not have any impact on our genes; and
  • It breaks down in the body shortly after it is taken into our cells.

Are the vaccines effective? I’ve heard about breakthrough cases.

All approved and authorized COVID-19 vaccines demonstrated efficacy (range 65 to 95%) against symptomatic lab-confirmed COVID-19 and high efficacy against COVID-19 severe enough to require hospitalization or death. Breakthrough cases have occurred in vaccinated individuals but symptoms are milder and the need for hospitalization in vaccine recipients is rare. The vast majority of patients who are hospitalized in the US are unvaccinated. The Advisory Committee on Immunization Practices (ACIP) states no preference for any of the three authorized vaccines.

The following table compares the three COVID-19 vaccines currently available in the U.S. It is important to note that most vaccine side effects from all three vaccines resolved in 1-2 days post-vaccination:

  Pfizer Moderna J&J
Number of shots 2 2 1
Days between shots 21 28 NA
Contains live virus No No No
Contains latex No No No
Contains preservatives No No No
Age authorized 5 years and older 18 years and older 18 years and older
Effectiveness preventing hospitalization & death 100% 100% 100%
Common side effects

injection site pain



muscle pain

joint pain

injection site pain



muscle pain

joint pain

injection site pain



muscle pain

What’s the difference between an additional dose and a booster?

An additional dose is recommended for people who do not have an adequate response to their original vaccine series due to conditions or medications that suppress their immune system. Getting an additional dose can help these individuals make more antibodies and protect against COVID-19.

A booster refers to another dose of a vaccine that is given to someone who built enough protection after vaccination, but then that protection decreased over time (this is called waning immunity).

So far, reactions reported after the additional dose or booster were similar to that of the two-dose series: fatigue and pain at injection site were the most commonly reported side effects, and overall, most symptoms were mild to moderate. However, as with the two-dose series, serious side effects are rare, but may occur.

Do I need an additional dose of vaccine?

COVID-19 vaccines have been shown to provide excellent protection against severe disease, hospitalization and death, and to decrease the spread of the virus. The U.S. Food and Drug Administration (FDA) expanded the emergency use authorizations for the Pfizer-BioNTech and Moderna vaccines to allow for the administration of an additional dose of these vaccines to individuals who may not have generated high levels of immunity from their two doses because they are immunocompromised. The Centers for Disease Control and Prevention (CDC) is recommending that moderately to severely immunocompromised people receive an additional dose.

Yale’s COVID-19 Vaccine Program is providing third doses to those identified as eligible by the CDC. Learn more about whether or not you qualify for an additional dose of an mRNA vaccine

Who is eligible for a booster dose of COVID-19 vaccines?

The CDC has recommended booster doses of the Pfizer, Moderna and Johnson & Johnson vaccines for all adults to increase protection against infection with COVID-19. 

Individuals who initially received the Pfizer or Moderna vaccines should get a booster if they are:

  • 6 months or more from the second shot in their initial vaccination series, and
  • 18 years of age or older.

All individuals who initially received the J&J vaccine should get a booster if they are:

  • two months or more from their initial vaccination and
  • 18 years of age or older.

In its recommendations, the CDC allows individuals who are eligible to choose which of the three vaccines they will receive as a booster shot. Many individuals will choose the same vaccine for their booster shot that they originally received, especially if they had no problems with that vaccination. CDC’s recommendations now allow for this type of mix and match dosing for booster shots.

If you have any questions about which vaccine you should choose for your booster, you should consult with your health care provider.

More information:

  • You will be asked to attest that you meet criteria for a booster dose at the time of scheduling. 
  • The CDC confirmed that it continues to consider individuals to be “fully vaccinated” two weeks after receiving two doses of the Pfizer or Moderna vaccines or one dose of the J&J vaccine.
  • Those who have completed a non-U.S. vaccine series may be eligible for a booster. Call the CCRL at 203-432-6604, if you need additional guidance. 

Are the vaccines safe?

COVID-19 vaccines are safe and effectiveThe Pfizer vaccine was approved by the FDA for those 16 and older, while the Modern and Johnson & Johnson vaccines were granted Emergency Use Authorization (EUA). The Pfizer vaccine continues to have EUA for those 5 years of age and older.  Learn more about EUAs in this video

COVID-19 vaccines were evaluated in tens of thousands of participants in clinical trials. While they may be new, the authorized COVID-19 vaccines have been tested in large clinical trials and the technology used (messenger RNA) has been in development for over a decade. An independent group of experts from the CDC’s ACIP, have found these vaccines to be safe and effective.

Over a hundred million people in the United States have received COVID-19 vaccines since they were authorized for emergency use by FDA. These vaccines have undergone and will continue to undergo the most intensive safety monitoring in U.S. history. This monitoring includes using both established and new safety monitoring systems to make sure that COVID-19 vaccines are safe. Learn more about COVID-19 vaccine safety monitoringWhile no medication or vaccine is without side effects, the COVID-19 vaccines are similar to other approved vaccines and side effects tend to be mild or moderate and last a couple of days. There have been a very limited number of allergic or other severe adverse reactions with these vaccines.

Should you have more questions, please contact a healthcare provider for more information.

Why is the FDA’s full approval of the Pfizer COVID vaccine so important?

The FDA’s “full approval” of this vaccine is a milestone as we continue to battle the COVID-19 pandemic. Better known to date as the Pfizer COVID-19 vaccine, it will now be marketed as Comirnaty for the prevention of COVID-19 disease in individuals 16 years of age and older. Comirnaty, which has the same formulation as the EUA vaccine, will continue to be administered as a series of two doses, three weeks apart. 

While this vaccine and other COVID-19 vaccines have met the FDA’s rigorous scientific standards for EUA, this first “fully FDA-approved” COVID-19 vaccine gives us even more confidence that this vaccine meets the highest standards for safety, effectiveness, and manufacturing quality that the FDA requires of an approved product.

While over 360 million COVID-19 vaccine doses have already been administered in the U.S, there are millions of unvaccinated individuals who should now feel more confident about getting vaccinated with the Pfizer COVID-19 vaccine.

What does “full approval” mean?

“Fully FDA-approved” vaccines undergo the agency’s standard process for reviewing the quality, safety and effectiveness of medical products. For all vaccines, the FDA evaluates data and information included in the manufacturer’s submission of a biologics license application (BLA). A BLA is a comprehensive document that is submitted to the agency providing very specific requirements. For the Pfizer COVID-19 vaccine, the BLA builds on the extensive data and information previously submitted that supported the EUA, such as preclinical and clinical data and information, as well as details of the manufacturing process, vaccine testing results to ensure vaccine quality, and inspections of the sites where the vaccine is made. The FDA conducts its own analyses of the information in the BLA to make sure the vaccine is safe and effective and meets the FDA’s standards for approval. 

The vaccine also continues to be available under EUA, including for individuals 12 through 15 years of age and for the administration of a third dose in certain immunocompromised individuals.

What do I need to know about myocarditis and pericarditis associated with mRNA vaccines (Pfizer and Moderna)?

The CDC has received reports of a small number of cases of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the lining around the heart) after vaccination with an mRNA COVID-19 vaccine. These rare cases have occurred most often in adolescent and young adult men aged 16 and older. Symptoms include chest pain and shortness of breath and occur several days after vaccination, more commonly after the second dose. Most cases are mild, and individuals recover often on their own or with minimal treatment. The FDA has updated the vaccine fact sheet to include a warning about the potential of this rare side effect, because it is very important that anyone with these symptoms seek medical care.  The CDC continues to emphasize that the benefits of vaccination far outweigh the risk and COVID-19 infection has much higher rates of myocarditis and pericarditis than vaccines.  Learn more about myocarditis and pericarditis associated with mRNA vaccines on the CDC’s website.

What do I need to know about the rare and serious events associated with the Janssen/J&J COVID-19 Vaccine? 

Blood Clots with Low Levels of Platelets 

A rare and serious adverse event (blood clots with low platelets) has been associated with the Janssen/J&J vaccine. This adverse event is very rare affecting about 7 per 1 million vaccinated women between 18 and 49 years old. The rate is even lower in other populations. Women younger than 50 years old should be aware of the rare but increased risk of this adverse event and that there are other COVID-19 vaccine options available for which this risk has not been seen. CDC and FDA will continue to monitor the safety of all COVID-19 vaccines

Women between 18 and 49 years old who received the Janssen/J&J vaccine are encouraged to follow this guidance:

If you have received the J&J/Janssen vaccine be aware of possible symptoms of a blood clot with low platelets within three weeks of receiving the vaccine. These include:

  • Severe or persistent headaches or blurred vision
  • Shortness of breath
  • Chest pain
  • Leg swelling
  • Persistent abdominal pain
  • Easy bruising or tiny blood spots under the skin beyond the injection site

Seek medical care right away if you develop one or more of these symptoms.

Guillain Barre Syndrome

Guillain Barre syndrome (a neurological disorder in which the body’s immune system damages nerve cells, causing muscle weakness and sometimes paralysis) has occurred in some people who have received the Janssen/J&J COVID-19 Vaccine. In most of these people, symptoms began within 42 days following receipt of the Janssen/J&J COVID-19 Vaccine. The chance of having this occur is very low. You should seek medical attention right away if you develop any of the following symptoms after receiving the Janssen/J&J COVID-19 Vaccine:

  • Weakness or tingling sensations, especially in the legs or arms, that’s worsening and spreading
  • to other parts of the body
  • Difficulty walking
  • Difficulty with facial movements, including speaking, chewing, or swallowing
  • Double vision or inability to move eyes
  • Difficulty with bladder control or bowel function

Can receiving the vaccination give someone COVID-19?

No. The vaccine does not contain live or dead versions of the virus, so it cannot give someone COVID-19.

What are possible side effects of the vaccine?

Common side effects include:

  • sore arm or pain and redness at the injection site
  • swollen lymph nodes in the underarm on the same side as the injection site
  • chills or fever
  • fatigue, body aches or feeling run down
  • headache
  • nausea, vomiting or diarrhea in the first 72 hours

Symptoms typically go away on their own within 1-2 days  and can be relieved with over the counter medications. Swollen lymph nodes may persist up to about 10 days. These side effects are a sign that your immune system is working to develop antibodies to COVID-19. 

Should I be concerned about a severe reaction to the COVID-19 vaccine?

Severe adverse reactions were uncommon during COVID-19 vaccine trials. They occurred more frequently after the second dose. All recipients will be monitored for at least 15 minutes after vaccination for possible immediate hypersensitive reactions. Read more about possible side effects of the COVID-19 vaccine.

How can I tell if side effects are from the vaccine or from actual COVID-19 infection?

The vaccine does not cause respiratory symptoms (cough/runny nose) or a loss of taste or smell, which are sometimes seen with a true COVID-19 infection. For these symptoms or anything apart from mild side effects, you should contact your provider for medical advice and possible COVID-19 testing.

How long will the vaccine protect those that receive it?

Pfizer reports that the vaccine is ninety-percent effective. Moderna reports that their vaccine is ninety-four-percent effective. While the studies haven’t indicated how long protection will last, the FDA predicts it to be effective for several months and possibly a year. Vaccine experts are continuing to study the virus and vaccine to learn more.

Are you immune to COVID-19 after recovering from it?

The extent to which antibodies that develop in response to SARS-CoV-2 infection are protective is still under study. If these antibodies are protective, it’s not known what antibody levels are needed to protect against reinfection. Therefore, even those who previously had COVID-19 can and should receive the COVID-19 vaccine.

My test for antibodies is positive, do I still need to be vaccinated?

Blood tests for antibodies to SARS-CoV-2 will not be accepted in place of vaccination. At this time, antibody tests cannot be interpreted as proof of immunity to COVID-19 and are not recommended post vaccination. People with positive antibody tests who are not fully vaccinated will not be exempted from public health requirements including the daily health check and weekly testing.

Why should someone get the vaccine even though they are doing other things such as wearing a mask, washing their hands often and practicing physical distancing?

Stopping a pandemic requires using all the tools available. Vaccines work with your immune system so your body will be ready to fight the virus and reduce or eliminate illness if you are exposed. Other steps, like covering your mouth and nose with a mask and staying at least six feet away from others, help reduce your chance of being exposed to the virus or spreading it to others.

How can I protect my unvaccinated child who is too young to receive a COVID-19 vaccine?

The following recommendations from the CDC are the best ways to protect your child, or a child you care for, who cannot get vaccinated yet:

  • Get vaccinated yourself. COVID-19 vaccines reduce the risk of people getting and spreading COVID-19.
  • If your child is 2 years and older, make sure that your child wears a mask in public settings and takes other actions to protect themselves.
    • To set an example, you also might choose to wear a mask.
  • If your child is younger than 2 years or cannot wear a mask, limit visits with people who are not vaccinated or whose vaccination status is unknown and keep distance between your child and other people in public.

Do the COVID-19 vaccines protect against the new variants of the virus?

Viruses constantly change through mutation, and new variants of a virus are expected to occur over time. Sometimes new variants emerge and disappear. Other times, new variants emerge and persist. Multiple variants of the virus that causes COVID-19 have been documented in the United States and globally during this pandemic, primarily from the United Kingdom, South Africa and Brazil. These variants seem to spread more easily and quickly than other variants, which may lead to more cases of COVID-19.

Scientists are working to learn more about how easily they spread, whether they could cause more severe illness, and whether currently authorized vaccines will protect people against them. So far, studies suggest that antibodies generated through vaccination with currently authorized vaccines recognize these variants. This is being closely monitored by the Centers for Disease Control and Prevention and more studies are underway.

For these reasons, it is essential to limit the spread of the virus by continuing to wear a mask that covers your mouth and nose, keep physical distance, wash your hands, get regularly tested, participate in contact tracing, and get the COVID-19 vaccine and booster when you are eligible. 

What about the Delta variant?

The Delta variant is a novel mutated strain of the original COVID virus. It was originally detected in India in December of 2020.  The Delta variant is now the predominant strain in the US and Connecticut.  

Key this to know: 

  • The Delta variant is highly contagious, and is at least twice as contagious as previous strains. 
  • The Delta variant may cause more severe illness in unvaccinated people, compared to previous strains. 
  • Unvaccinated persons are still the greatest concern: they remain at highest risk of severe illness and of spreading the virus to others. 
  • People who are vaccinated can become infected with COVID-19 (this is called breakthrough infection), however, these infections are generally mild.   
  • Vaccinated people who are symptomatic with COVID-19 breakthrough infections can spread the virus. However, they are thought to be contagious for shorter time than unvaccinated persons.  

What you can do: 

  • Vaccination continues to be one of the most important things you can do protect yourself, your family, and community.  
  • Currently available vaccines are strongly protective against COVID, including the Delta variant, at reducing serious illness, including risk of hospitalization and death.  
  • Masks are essential as well. Given current understanding of the contagious nature of the Delta variant, vaccine effectiveness, and current vaccine coverage, masking indoors is also key to reducing spread of COVID at this time.  

What about the Omicron variant?

A new variant of concern, Omicron, has emerged and been detected in the United States. Our university public health experts and researchers are actively evaluating the characteristics and impacts of the Omicron variant and regularly exchange information with state, national, and international public health organizations. However, because we do not yet know for certain how this new variant will respond to vaccines, how transmissible it may be, or what symptoms it causes, we must be continue to follow all health and safety guidelines.

Does the flu vaccine also protect you from COVID-19?

No. The coronavirus (COVID-19) and the flu are completely different. The flu vaccine does not protect you from becoming infected with COVID-19, so you should get both vaccines. One of the benefits from getting a flu vaccine in addition to the COVID-19 vaccine, is that a recent study suggested that people who received flu vaccine faced a lower risk for being hospitalized if they got COVID-19.

How long should an individual wait to receive the COVID-19 vaccine after receiving another vaccine?

COVID-19 and other vaccines may now be administered without regard to timing of other vaccines. This includes simultaneous administration of COVID-19 and other vaccines on the same day.

What does it mean to be “fully vaccinated?”

People are considered fully vaccinated:

  • 2 weeks after their second dose in a 2-dose series, like the Pfizer or Moderna vaccines, or
  • 2 weeks after a single-dose vaccine, like the Johnson & Johnson vaccine

If it has been less than 2 weeks since your shot, or if you still need to get your second dose, you are NOT fully protected.

I am now fully vaccinated. What does the CDC advise?

CDC issued guidance allows individuals who have been fully vaccinated to resume certain pre-pandemic activities. More guidance will likely follow if we continue to be diligent in adhering to health and safety practices and if we get vaccinated. However, while on campus, it is still important to follow our health and safety guidelines.

Can we all stop wearing masks once vaccinations are given?

The CDC provides guidance on masking for the general populatiopn and Environmental Health and Safety provides the most updated mask guidance for the Yale community.   

Does this mean you can assume that anyone you see on campus wearing a mask is unvaccinated?

The simple answer is “no.” There are many reasons why someone who is fully vaccinated may choose to continue to wear a mask in settings where they may no longer be required. Some may have been advised or feel that they remain at greater risk for COVID-19 infection despite being vaccinated. Some may have concerns for family members who are unable to be vaccinated at this time. Still others have recognized the benefits of mask-wearing beyond preventing transmission of COVID-19, such as reducing exposure to allergens or to colds and other respiratory illnesses. And some simply continue to feel more comfortable wearing a mask in public settings. Most importantly, we should be supportive and respectful of our mask-wearing colleagues and not make assumptions about their reasons for wearing a mask.

Updated December 3, 2021