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Frequently Asked Questions about COVID-19 Vaccines and Vaccinations for Press and Media

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General Questions

When will a vaccine for COVID-19 be available?

On December 11, 2020, emergency use authorization (EUA) by the U.S. Food and Drug Administration (FDA) was granted to the Pfizer-BioNTech COVID-19 vaccine, with first allocations shipped on December 12, 2020. UC San Diego received its first shipment of the Pfizer vaccine (2,925 doses) on December 15, 2020, with the first inoculations of health care workers beginning on December 16, 2020. An additional shipment of 975 doses from Pfizer arrived on December 21.

The FDA granted an EUA for the Moderna COVID-19 vaccine on December 18, 2020. UC San Diego Health received its first shipment of 5,500 doses of the Moderna vaccine on December 22, 2020. The Moderna vaccine has been incorporated into the ongoing vaccine distribution plan at UC San Diego Health, with inoculations continuing seven days a week, dependent upon supplies and circumstances.

AstraZeneca and Janssen are also engaged in Phase III clinical trials for COVID-19 vaccines. (UC San Diego Health has been a testing partner for three of the four leading vaccines in Phase III clinical trials: Moderna, AstraZeneca and Janssen.)

At last count, researchers worldwide were reported to be testing 64 vaccines in clinical trials on humans, with 18 in final stages of testing, and at least 85 preclinical vaccines under investigation in animals. All strive to produce a safe, effective coronavirus vaccine within the next year or so. The more vaccines that pass muster, the faster and more broadly people and populations can be vaccinated.

What is an emergency use authorization?

Drugs and vaccines must be approved by the FDA to ensure only safety and efficacy before they are made available to the American public. In situations where there is sound scientific evidence to believe a drug or therapy is safe and is likely to treat or prevent disease, the FDA may authorize its use before complete, definitive proof of efficacy has been shown, especially for diseases that cause high mortality and present a major public health threat, such as COVID-19. You can learn more here.

How do the vaccines work?

Different vaccines are employing different methodologies, both well-tested and new. The Pfizer and Moderna vaccines are messenger RNA (mRNA) vaccines, and represent a newer approach. These vaccine give our cells the instructions to produce a harmless piece of the virus' characteristic spike protein. Our immune systems recognize the spike protein as "foreign" and build an immune response against it.  Then, if our bodies are exposed to the SARS-CoV-2 virus in the future, our immune systems are already prepared to help prevent us from getting sick.

Messenger RNA vaccines do not alter your DNA. The mRNA from a COVID-19 vaccine does not enter the nucleus of the cell, where DNA is kept.  mRNA vaccines have proven to be quicker to develop than traditional DNA-based vaccines, but they can be more difficult to store. The Pfizer vaccine must be stored at ultra-low temperatures.  The Pfizer vaccine is the first such mRNA vaccine to be approved for human use. Of note: the Moderna vaccine does not require ultra-low temperatures for storage and can be kept in a refrigerator for up to a month.

The AstraZeneca vaccine, which has not yet reached the point of EUA submission and FDA review, is made from a weakened version of an adenovirus (which causes the common cold) derived from chimpanzees and modified so it cannot replicate in humans. The approach is similar to the strategy used in previous human vaccine trials for prevention of Middle East Respiratory Syndrome (MERS), a coronavirus closely related to COVID-19.

The adenovirus encodes the full-length surface glycoprotein of SARS-CoV-2 that gives the novel coronavirus its characteristic spikes. These spike proteins are used by the virus to fuse with host cell membranes. When the vaccine adenovirus binds to host cells, the SARS-CoV-2 genetic material it contains gets made into proteins that allow for an immune response and ideally, the development of neutralizing antibodies.

The Janssen vaccine, also not yet submitted to the FDA for review, is in Phase III human clinical trials. It employs a deactivated adenovirus that is modified to carry the characteristic spike protein as well. The modified cold virus introduces the vaccine into cells where they translate the genes for the spike protein.  As with the other vaccines described, these spike proteins can't cause disease without the rest of the SARS CoV-2 virus, but they can train the human immune system to create neutralizing antibodies to block the targeted pathogen, building immunity against SARS CoV-2.

The Pfizer, Moderna and AstraZeneca vaccines all require two injections, roughly 21 to 28 days apart. The Janssen vaccine is being evaluated as a single shot.

What are neutralizing antibodies?

Regardless of approach, the goal of all vaccines is to prompt the human immune response to generate neutralizing antibodies (NAb). These are proteins made by B-cells in bone marrow that circulate in the blood to help fight off infections and provide protection against future infection. They are disease-specific. Each type of NAb recognizes and targets a specific pathogen, such as SARS-Cov-2. They work by blocking a pathogen's ability to enter a cell.

Once created, either by natural disease exposure or vaccine, some NAbs produce lifelong immunity to certain infections, such as measles and polio. Others generate immunity that may last for decades, such as hepatitis B, tetanus and mumps. Some NAbs are short-lived, persisting in the blood for only a few months or years after infection or vaccine. The seasonal flu vaccine is an example of a short-lived NAb immune response.

What are the potential known side effects of current COVID-19 vaccines?

The most frequently reported adverse reactions by Pfizer clinical trial participants were injection site reactions (84 percent), fatigue (63 percent), headaches (55 percent), muscle pain (38 percent), joint pain (24 percent) and fevers (14 percent).

None are unique to the COVID-19 vaccine.

The seasonal flu shot, for example, can cause fever, fatigue and soreness at the injection site. The highly effective shingles vaccine can induce shivering, muscle pain and upset stomach, among other symptoms.

The important points to remember are: 1.) Less than 0.5 percent of COVID-19 clinical trials participants reported serious adverse effects. 2. The adverse effects are temporary. 3. They appear more likely after the second dose of the vaccine.

Transient discomfort, perhaps requiring some recipients to miss a day or two of work, is a small price for substantial protection against the much more severe outcomes of a COVID-19 infection.
Published side effects data on the Moderna vaccine are more limited at the moment, but researchers report 10 percent of clinical trial participants experienced severe fatigue, 9 percent severe muscle ache, 5 percent severe joint pain and 5 percent severe headache. This data will be updated over time.

Again, in both clinical trials adverse reactions were usually mild to moderate in intensity and resolved within a few days of vaccination. If needed, most post-vaccination symptoms can be treated with pain relievers and fever-reducing medications.

Pfizer notes that there is a "remote chance" that its vaccine could cause a severe allergic reaction, which would usually occur within a few minutes to one hour after getting a dose of the vaccine. Signs of a severe allergic reaction can include:

  • Difficulty breathing
  • Swelling of your face and throat
  • A fast heartbeat
  • A bad rash all over your body
  • Dizziness and weakness

If you have a history of allergic reactions, please inform your vaccination provider before inoculation. If you have allergies serious enough to require carrying an EpiPen or similar device to counter a severe allergic response, please make sure to have it with you at time of vaccination. The CDC said it is reviewing the handful of reported cases of anaphylaxis in people given the Pfizer vaccine.

These side effects may not be all of the possible side effects of the Pfizer vaccine. Safety monitoring for adverse reactions due to vaccination continue, and will continue through the on-going clinical trials and vaccination programs for months and years to assure any less common or delayed problems are identified and can be remedied.

Can I get COVID-19 from the vaccine?

No, none of the current leading vaccines (Pfizer, Moderna, AstraZeneca, Janssen) contain whole SARS-CoV-2 virus. They cannot give you COVID-19.

Who will be vaccinated first?

Given the current realities of vaccine development and production, distribution of the first rounds of doses in a smart, efficient and equitable manner that is based on science and best practices will necessarily involve myriad complications and considerations. The goal, however, is quite simple and straightforward: We want to offer a COVID-19 vaccine to everyone as soon as possible.

There are a lot of ways to achieve success, and a large team of dedicated experts and professionals at UC San Diego Health has spent many days and weeks exploring them all.

Generally speaking, vaccine distribution will be coordinated through the California Department of Public Health and local public health departments, and governed by recommendations from the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices (ACIP), with guidance evolving as circumstances dictate.  

Initially, ACIP recommends that health care workers in the patient care environment receive the first available vaccinations, along with residents of nursing homes and long-term care facilities, who are at higher risk of COVID-19 complications and death.

Specific distribution will depend upon doses available, with an emphasis on vaccinating first those health care workers with greatest exposure to COVID-19 patients, such as emergency department staff, trauma staff, respiratory therapists and personnel with face-to-face patient care in urgent care clinics. 

ACIP is scheduled to vote again later this month or next on recommendations for the next group of persons who should receive the COVID-19 vaccine. It is likely these recipients will be essential workers, such as police, firefighters, grocery store workers and the like, followed by persons over the age of 65 and those with underlying health conditions that put them at higher risk of a severe outcome from a COVID-19 infection, such as individuals with diabetes, obesity or cancer.

Am I immediately protected from COVID-19 after vaccination?

No. Data from the Pfizer and Moderna clinical trials show that protection from COVID-19 appears to begin approximately two weeks after the first injection.  While protection was seen after the first shot, it is strongly recommended that everyone complete the two-dose regimen to fully boost immunity and stronger, longer lasting protection. Going forward, the timing between the first and second shots may change, based on new findings, but for now it's recommended to get the two shots according to each vaccine's schedule.

Why should I get the vaccine when it becomes available to me?

Since January, less than a year after the novel coronavirus first emerged in the U.S., nearly 16 million Americans have been diagnosed with COVID-19 (including 1.5 million in California alone) and more than 295,000 people have died, according to the CDC.

That mortality figure for COVID-19 is more than 11 times the number of people who died from the flu in the U.S. last year, and more than the last five flu seasons combined. Many who died were young and did not have underlying medical conditions. Many people who survived COVID-19 are still living with debilitating breathing, cardiac, kidney and neurological problems, even months after recovering from the immediate infection. We do not yet know how their long-term health may be affected.

A COVID-19 vaccine is your best chance to prevent becoming infected, and perhaps experiencing a severe outcome. Even if you are not concerned for your own health, a vaccination will help prevent you from spreading the virus to others, perhaps those more vulnerable or who haven't yet been or cannot be vaccinated. No vaccine is 100 percent effective, and so the more people in our communities who become vaccinated, the less the virus will circulate among us and the better protected we all will be.

Why should I get vaccinated if other measures, such as wearing masks and social distancing, help prevent the spread of the coronavirus?

Stopping a pandemic requires using every tool and measure available. Vaccines work with your immune system so your body will be ready to fight the virus 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. Contact tracing measures like CA NOTIFY employ your smart phone to individually alert you to potential virus exposure. Altogether, getting vaccinated, following the CDC's recommendations to protect yourself and others and other efforts offer the best combined protection from COVID-19.

What if I have already had COVID-19, do I need to be vaccinated?

Yes, but it is reasonable to wait and get vaccinated 90 days after your COVID-19 illness has resolved because current research suggests people develop some natural, albeit temporary, immunity.

Nonetheless, it is recommended that you do get vaccinated eventually for a number of reasons.


  1. There have been cases reported of COVID-19 survivors becoming re-infected.
  2. There is not enough information yet to say if or for how long after infection someone is protected from getting COVID-19 again; this is called natural immunity. Early evidence suggests natural immunity from COVID-19 may not last very long, but more studies are needed to better understand this.
  3. Given the severe health risks associated with COVID-19, it is wise to seek as much protection from infection as possible.

Who is excluded from receiving the vaccine?

You should not receive the Pfizer vaccine if you had a severe allergic reaction to any previous dose of this vaccine or if you had a severe allergic reaction to any ingredient in this vaccine.

Currently, children are the only demographic group expressly prohibited from receiving the vaccine. Clinical trials have begun to assess safety and efficacy of some COVID-19 vaccines in children as young as 12. Depending upon results, this age restriction could change in the future. 

There is minimal to no data on how these vaccines affect other specific groups, such as pregnant and breastfeeding women or persons with severely immunocompromised systems.  Research is ongoing. Notably, the American College of Obstetricians and Gynecologists has issued an advisory that recommends COVID-19 vaccines should not be withheld from pregnant individuals who meet criteria for vaccination based on ACIP-recommended priority groups.

Individuals in these groups or with specific concerns should contact their health care provider for more information and guidance.

What are the ingredients in the Pfizer vaccine?

The Pfizer vaccine includes the following ingredients: mRNA, lipids ((4-hydroxybutyl)azanediyl)bis(hexane-6,1-diyl)bis(2-hexyldecanoate), 2 [(polyethylene glycol)-2000]-N,N-ditetradecylacetamide, 1,2-Distearoyl-sn-glycero-3-phosphocholine, and cholesterol), potassium chloride, monobasic potassium phosphate, sodium chloride, dibasic sodium phosphate dihydrate, and sucrose.

What are the ingredients in the Moderna vaccine?

The Moderna COVID-19 Vaccine contains the following ingredients: messenger ribonucleic acid (mRNA), lipids (SM-102, polyethylene glycol [PEG] 2000 dimyristoyl glycerol [DMG], cholesterol, and 1,2-distearoyl-sn-glycero-3-phosphocholine [DSPC]), tromethamine, tromethamine hydrochloride, acetic acid, sodium acetate, and sucrose.

What are the differences between the Pfizer and Moderna vaccines?

Apart from storage requirements, the vaccines are essentially the same, both in basic approach and in how they are dispensed. There are some minor variations:

  • The Pfizer EUA is for people aged 16 and older; Moderna’s applies to people 18 and older, although the company has recently begun testing its vaccine in 12- to 17-year-olds.
  • Each dose of Pfizer’s vaccine contains 30 micrograms of vaccine: Moderna’s vaccine contains 100 micrograms—a little more than three times as much vaccine per person as Pfizer, but with similar efficacy.
  • The Pfizer vaccine is shipped in five-dose vials, but due to overfill, health systems are often extracting a sixth dose per vial. Moderna’s vaccine comes in 10-dose vials.

What if I have preexisting conditions? Should I get vaccinated?

If you have any preexisting medical conditions or other concerns, please consult fact sheets available from the vaccine makers, the CDC or FDA and/or discuss with your health care provider.

Vaccination Process

How do I receive the vaccine?

The actual injection is similar to a flu shot, given in the deltoid muscle or top shoulder. Since there is a reasonable likelihood that the vaccine may cause transient side effects, such as fever or achiness in muscles and joints, you should be expect to need a day or so of rest and recovery as these effects diminish and pass.

What should I mention to my vaccination provider before I receive any shots?

Pfizer recommends you inform your vaccination provider about all medical conditions, including if you:
·      have any allergies
·      have a fever
·      have a bleeding disorder or are on a blood-thinner
·      are immunocompromised or are on a medicine that affects your immune system
·      are pregnant or plan to become pregnant
·      are breastfeeding
·      have received another COVID-19 vaccine

Should I do anything in preparation for being vaccinated?

Schedule smartly. Because clinical trial data suggest the mRNA vaccines are likely to produce temporary flu-like symptoms in many people, you should schedule your shots so that you have the opportunity to rest and recover for a day or two afterward. These transient side effects should not be alarming. They are a good sign that your body's immune response system is building future protection.

Do not pre-medicate before vaccination to prevent side effects. While over-the-counter medications, such as aspirin, acetaminophen (e.g. Tylenol) and ibuprofen (e.g. Motrin) can help ease post-vaccination discomfort, taking them before getting vaccinated may reduce a vaccine's ability to work or blunt the immune response.


How do I report if I have a problem or bad reaction after getting a COVID-19 vaccine?

Both the CDC and FDA encourage the public to report possible side effects (called adverse events) to the Vaccine Adverse Event Reporting System (VAERS), which collects these data to look for adverse events that are unexpected, appear to happen more often than expected or have unusual patterns of occurrence

Vaccine recipients can also notify their health care provider, who is required to report certain adverse events following vaccination to VAERS.

The CDC has also implemented a smartphone-based tool called v-safe to check-in on people’s health after they receive a COVID-19 vaccine. When you receive your vaccine, you should also receive a v-safe information sheet telling you how to enroll in v-safe. If you enroll, you will receive regular text messages directing you to surveys where you can report any problems or adverse reactions you have after receiving a COVID-19 vaccine.

Do I need to wear a mask after being vaccinated?

Yes. You must continue to wear a mask, follow physical distancing, hand hygiene guidelines and all other recommendations and requirements set forth by local, state and federal public health agencies.

Are there other vaccines that can help prevent me from getting COVD-19?

No. The seasonal flu vaccine will not protect you from getting COVID-19, but it can prevent you from getting influenza at the same time as COVID-19. This may prevent a more severe COVID-19 illness. If you have not received your 2020-2021 seasonal flu shot, go here.

While it's not possible to say with certainty what will happen this winter, the CDC believes both flu viruses and SARS-CoV-2, the virus that causes COVID-19, will both be active and widespread. That means that getting a flu vaccine is more important than ever.

For the two-dose vaccines, can I get the first dose with one vaccine and the second dose with another vaccine?

No. There is no data on the safety and efficacy of mixing and matching COVID-19 vaccines. Both doses must be from the same manufacturer and, for now, received from the same vaccination provider.

For vaccines requiring two shots, what happens if the second shot is delayed due to lack of availability or another reason?

The period between the first and second doses is intended to allow the former to begin building immunity in the body to the novel coronavirus. Data from the Pfizer trial suggests the first dose establishes a robust protective response, approximately 70 percent effectiveness, with the second dose providing a booster to full efficacy. In the case of both Pfizer and Moderna vaccines, the estimated full effectiveness rate is 95 percent, based on clinical trial data. 

It is not known how the efficacy of the vaccine changes with longer intervals between injections. However, if there is a delay of a couple of weeks between doses, experts think the added time poses no safety or effectiveness issues.

Can I choose a particular vaccine or get vaccinated with more than one?

Not at this time. The goal is to vaccinate as many people as possible as quickly as possible. The type of vaccine offered will depend upon availability, which will likely be affected by multiple, changing factors.

It may be that some vaccines developed will not prove appropriate for certain persons, such as those with immunocompromised systems, and in those cases we will update guidance.

Generally speaking, all of the vaccines expected to be approved in the coming months will have similar rates of efficacy; one will not be significantly "better" than another, and receiving multiple, different vaccinations could prove unsafe. It should be avoided. Moreover, one person-one vaccine means available vaccines can go to more people in the population more quickly, maximizing societal benefits and helping stop the pandemic faster.

The most important consideration is to get vaccinated as soon as the opportunity arises.

How long will the COVID-19 vaccine last? Will it require an annual shot like the flu?

That remains to be seen. The novel coronavirus SARS-CoV-2, which causes COIVD-19, emerged only late 2019. While much has been learned since then — and development of vaccines has occurred with unprecedented speed — there is much about the virus that remains a mystery, including how long vaccine protection lasts and how frequently SARS-CoV-2 mutates, necessitating vaccine adaptation. Influenza mutates routinely and often, requiring annual, reformulated vaccines.

SARS-CoV-2 also mutates regularly, acquiring about one new mutation in its genome every two weeks. Many mutations do not fundamentally change the nature or behavior of the virus; some can actually make a virus less virulent. Nonetheless, a recently identified variant has prompted concern that it is more rapidly transmissible than other circulating strains. It is not yet clear whether the current COVID-19 vaccines, approved or in trials, are equally effective against new variants, though vaccine makers say they appear so. Research is ongoing.

There are ways to potentially make protection following vaccination more durable than just protection created by natural infection. The use of an adjuvant — an ingredient used in some vaccines to create a stronger immune response, for example, or subsequent booster doses. These enhancement strategies may be particularly important for vulnerable populations, such as the elderly and those with underlying diseases, who are at particular risk of severe Covid-19 but who may be less likely to develop a strong, protective immune response to a vaccine. Further research and development will be needed.

Will getting vaccinated cause me to test positive on COVID-19 viral tests?

Vaccines currently in clinical trials in the United States won't cause you to test positive on virus detection tests, which are used to determine if you have a current infection. If your body develops an immune response, which is the goal of vaccination, there is a possibility you may test positive on some antibody tests. Antibody tests indicate you have had a previous infection and that you may currently have some level of protection against the coronavirus. Experts are currently looking at how COVID-19 vaccination may affect antibody testing results.

Does immunity after getting COVID-19 last longer than protection from COVID-19 vaccines?

The protection someone gains from having an infection (called natural immunity) varies depending upon the disease. It also varies from person to person. Since this virus is new, we don't know how long natural immunity lasts. Some early evidence — based on some people — seems to suggest that natural immunity lasts only a few months.

We won't know how long immunity lasts after vaccination until we have more data on how well these vaccines work over time.

Both natural immunity and vaccine-induced immunity are important aspects of COVID-19 that experts are trying to learn more about.

What percentage of the population needs to get vaccinated to achieve herd immunity to COVID-19?

Experts do not yet know what percentage of people would need to get vaccinated to achieve herd immunity to COVID-19. Herd immunity is a term used to describe when enough people have protection — either from previous infection or vaccination — that it is unlikely a virus or other pathogen can spread and cause disease. As a result, everyone within the community is protected even if some people don't have any protection themselves. The percentage of people who need to have protection in order to achieve herd immunity varies by disease.

Based on current knowledge, it has been estimated that herd immunity to COVID-19 would mean 60 to 70 percent of the population to have antibodies to the disease, either through vaccination or previous infection. To achieve herd immunity without vaccines would require infection rates approaching 800,000 new cases per day over a nine-month period. The consequential effects of that approach would be devastating, both in terms of public health and the economy.

After vaccination, can I resume my normal, pre-pandemic life?

No. First, even after both doses, it will take your body at least two weeks to develop maximum immunity. Second, none of the current vaccines is 100 percent effective. There remains a reduced chance that you might be in the minority of vaccinated persons who can still become infected (or re-infected) with COVID-19, who may develop symptoms or unknowingly transmit the virus to others who have not been vaccinated or cannot be vaccinated. 

It is not yet known whether vaccinated persons can become infected with SARS-CoV-2, show no symptoms, and yet still transmit the disease to others. We will learn more about this over time.

For these reasons and others, prevention measures like wearing a mask, maintaining physical distance and proper hand hygiene will continue to be extremely important for the foreseeable future.

Where can I find additional vaccine information that is accurate and credible?

The CDC maintains a site specific to COVID-19 and offers a regularly updated source list. Pfizer has created its own information site for health care professionals and for non-health care professionals. It includes a fact sheet for recipients and caregivers. Similarly, Moderna has an information site and its own fact sheet.

UC San Diego Health maintains its own COVID-19 information site here. These sites are useful tools for learning more, but they do not replace discussions with your own health care provider.


Apart from the actual vaccine, are there concerns that there will be shortages of the ancillary supplies needed to vaccinate, such as syringes?

No. The vaccine makers will provide all necessary materials in self-contained kits with the vaccine drug.

Why do some vaccines have to be stored at ultra-low temperatures?

The Pfizer and Moderna vaccines rely upon mRNA molecules, which are very fragile. To keep them intact, the molecules are encased in tiny droplets of fat, which must be stored at very low temperatures to maintain their shape and shield the mRNA.

For the Pfizer vaccine, that means transporting and storing it at minus-70 degrees Celsius (-94 degrees Fahrenheit). For the Moderna vaccine, transport and storage temperatures must be minus 20 degrees C (-4 degrees F).

UC San Diego Health has multiple ultra-low cold freezers (minus-80 degrees C/-112 degrees F) capable of storing up to 100,000 vaccine doses at one time. Both the Pfizer and Moderna vaccines are thawed out before use, and usable for several days when stored in regular refrigerators.

The AstraZeneca and Janssen vaccines can be transported and stored at normal refrigerator temperatures.

What sort of requirements or certifications must the freezers meet?

These vaccine storage freezers must meet all regulatory requirements for storage of medications, including requirements of the California Department of Public Health and California Board of Pharmacy. They are electronically monitored with remote alarms for any diversions from the appropriate temperature range.

Where are these freezers kept? Do they require special security measures?

They are located in secured spaces at UC San Diego Medical Center in Hillcrest and at Jacobs Medical Center in La Jolla.

How will other vaccines, which don't require super-cold conditions, be stored?

We anticipate that other COVID-19 vaccines will be stored in the main pharmacy or appropriate refrigerators and -20C freezers at the same locations as the -80C freezers.

Updated: Dec. 28, 2020