iv. Vaccine Safety and Effectiveness

III. COVID-19 Vaccine FAQs

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iv. Vaccine Safety and Effectiveness


In addition to its regular rigorous standards, the FDA has issued assurances that it will not authorize a vaccine that was developed by sacrificing the standards for quality, safety and effectiveness that any other vaccine would need to meet. California also has a Scientific Safety Review Workgroup that will independently review the safety and effectiveness of any vaccine that the FDA approves.

Visit these pages to learn more about vaccine safety and approval:


An Emergency Use Authorization (EUA) by the FDA permits use of a vaccine after declaration by the Health and Human Services Secretary of an emergency situation leading to serious or life-threatening disease or condition. In order to grant an EUA, the potential benefits of the vaccine must outweigh the potential risks of the vaccine.

In the case of COVID-19 vaccines, the FDA has recommended that both full approval and emergency use authorization show at least 50% effectiveness. Safety requirements are identical for EUA and full FDA Biologic License approval, but full Biologic Licensed approval requires following subjects for a longer period of time to gauge safety and effectiveness.

The FDA has required that subjects vaccinated during COVID-19 vaccine trials must be followed for a median of two months. However, the FDA recommends that, following EUA, the stage 3 studies continue for sufficient time to allow application for a full Biologic License (full approval).

California also has a Scientific Safety Review Workgroup that independently reviews the safety and effectiveness of any vaccine that the FDA approves.

Recent DevelopmentsJohnson and Johnson (Janssen) Vaccine: On April 13, 2021, the FDA recommended a pause in administration of the Janssen vaccine due to a report of 6 cases of a syndrome, known as thrombosis plus thrombocytopenia syndrome or TTS. All six cases occurred in women age 18-48 and 1 person had died. On April 23,2021, after a 10-day pause of examination of all evidence related to those cases as well as 9 additional cases, which included 2 additional deaths, the CDC and FDA found that the vaccine’s known and potential benefits outweigh its known and potential risks in individuals 18 years of age and older. The TTS was shown to have a risk of 7 cases per 1 million doses of the Janssen vaccine administered to women ages 18 to 49. The risk for women older than 50 was .9 per 1 million doses. Compared to the risks for hospitalization and death from COVID-19, these risks were less than the risks from not being vaccinated for COVID-19. Both agencies recommended a resumption of administration of the Janssen vaccine, along with revision of the Fact Sheets for providers and recipients to include information about the risk of this syndrome, which has occurred in a very small number of people who have received the Janssen COVID-19 Vaccine. Visit the pages below to learn more:

Visit the pages below to learn more:


Most vaccines provide protection both for the person being vaccinated and for those with whom he or she comes in contact. In the case of COVID-19, Stage 1 and 2 clinical studies of the vaccines have established that the vaccine causes an immune response in the recipient. Stage 3 clinical studies have focused on how well the vaccine protects those who are vaccinated from developing symptoms and getting sick, and are required to show that the vaccine is effective in doing so. According to the CDC, people who do not show symptoms are less infectious to others than people who have symptoms.

Recent studies of how well vaccines reduce even asymptomatic infection from COVID-19 have been encouraging. So far, indications are that the mRNA vaccines reduce infections and the ability to transmit the virus between 60-80%. More studies need to be done to examine this issue. Until these studies are done, it is wise for people who have been vaccinated to continue to wear masks and socially distance around others who have not been vaccinated in order to provide the maximum protection.
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According to the CDC, vaccination should be offered to persons regardless of history of prior symptomatic or asymptomatic SARS-CoV-2 infection. Vaccination should be deferred until recovery from acute illness (if person had symptoms) and criteria have been met to discontinue isolation. There is no minimum interval to wait before vaccination; however, the CDC says, “Current evidence suggests reinfection uncommon in the 90 days after initial infection, and thus persons with documented acute infection in the preceding 90 days may defer vaccination until the end of this period, if desired.”


There has been no indication during clinical trials that persons with allergies have more severe reactions to the Pfizer or Moderna vaccines than other people. However, during the early vaccination of healthcare workers in the U.K., two workers both with a history of anaphylactic reactions (severe allergic reactions) to injected medications or vaccines did have anaphylactic reactions to the Pfizer vaccine and had to be treated for those reactions. A recent study by the CDC Advisory Committee on Immunization Practices (ACIP) reported that, as of 1/18/21, there had been a total of 71 cases of anaphylaxis following COVID-19 vaccination in the US.  The rate anaphylaxis was 5 per million shots for the Pfizer vaccine and 2.8 per million shots for the Moderna vaccine. For that reason the CDC has recommended that:

  • Severe allergic reaction (e.g., anaphylaxis) to any component of the Pfizer-BioNTech or Moderna COVID-19 vaccines is a contraindication to vaccination.
  • A severe allergic reaction to any vaccine or injectable therapy (intramuscular, intravenous or subcutaneous) is a precaution to vaccination at this time
  • Vaccine providers should observe patients after vaccination to monitor for the occurrence of immediate adverse reactions:
    • Persons with a history of anaphylaxis: 30 minutes
    • All other persons: 15 minutes

Appropriate medical treatment used to manage immediate allergic reactions must be immediately available in the event an acute anaphylactic reaction occurs following administration of the vaccine.

No cases of anaphylaxis were reported during clinical trials of the Johnson and Johnson vaccine.


According to the CDC, COVID-19 vaccine may be administered to persons with underlying medical conditions who have no contraindications to vaccination. Phase 2/3 clinical trials demonstrated similar safety and efficacy profiles in persons with underlying medical conditions, including those that place them at increased risk for severe COVID-19, compared to persons without comorbidities.

Persons with HIV infection or other immunocompromising conditions, or who take immunosuppressive medications or therapies, might be at increased risk for severe COVID-19. Data are not currently available to establish safety and efficacy of vaccine in these groups. These individuals may still receive COVID-19 vaccine unless otherwise contraindicated.


According to the CDC, there are no data on the safety of COVID-19 vaccines in pregnant women.

  • Animal developmental and reproductive toxicity (DART) studies are ongoing.
  • Studies in humans are ongoing and more planned.
  • According to the CDC, numerous pregnant women have now been vaccinated against COVID-19 without serious consequences.

mRNA vaccines and pregnancy

  • mRNA vaccines do not contain live virus, and they are degraded quickly by normal cellular processes and do not enter the nucleus of the cell. There are currently insufficient data to make conclusions about the safety of the vaccine in pregnant and lactating individuals and their infants
  • Johnson and Johnson adenovirus vaccine: pregnant women were not included in the clinical trials.

COVID-19 and pregnancy

  • Women who are pregnant are at increased risk of severe illness (ICU admission, mechanical ventilation and death)
  • Women who are pregnant might be at increased risk of adverse pregnancy outcomes, such as preterm birth

Pregnancy is one of the medical conditions listed by CDPH as qualifying for priority eligibility for vaccination. If a woman is pregnant, she may choose to be vaccinated. A discussion with her healthcare provider can help her make an informed decision.


From the CDC: Yes. People who want to get pregnant in the future may receive the COVID-19 vaccine.

Based on current knowledge, experts believe that COVID-19 vaccines are unlikely to pose a risk to a person trying to become pregnant in the short or long term. Scientists study every vaccine carefully for side effects immediately and for years afterward.

The COVID-19 vaccines are being studied carefully now and will continue to be studied for many years, similar to other vaccines. The COVID-19 vaccine, like other vaccines, works by training our bodies to develop antibodies to fight against the virus that causes COVID-19, to prevent future illness. There is currently no evidence that antibodies formed from COVID-19 vaccination cause any problems with pregnancy, including the development of the placenta. In addition, there is no evidence suggesting that fertility problems are a side effect of ANY vaccine. People who are trying to become pregnant now or who plan to try in the future may receive the COVID-19 vaccine when it becomes available to them.


None of the currently developed COVID-19 vaccines have the live SARS-CoV-2 virus. These vaccines cannot give you or anyone else COVID-19, nor do they make you contagious.


No. The vaccine will not cause you to test positive on viral tests for COVID-19, such as PCR tests or antigen tests. However, the vaccine likely will cause you to test positive for antibody tests (also called serology) since the vaccine helps build antibodies to COVID-19.


Effectiveness of the vaccines was reported for different endpoints, but each vaccine included data on effectiveness preventing any symptomatic infection, severe infection, hospitalization and death. Effectiveness was measured as a differences in infections between those who received the vaccine and those who didn’t (placebo) following a full regimen of the vaccine (2 doses in the case of Pfizer and Moderna and 1 dose in the case of Johnson and Johnson) and the prescribed follow-up period to achieve full protection.

Data are presented below: (Note: The Pfizer and Moderna vaccines had different cutoff ages for trials with older adjults. The Johnson and Johnson vaccine was tested at a later date and in different locations than the Pfizer and Moderna vaccines making head-to-head comparison invalid).

Vaccine Effectiveness Data

Vaccine

Type

Doses

Ages

Effectiveness*

References

 

 

 

 

Any Symptoms

Severe Symptoms

Hospitalization or Death

 

Pfizer

mRNA

2

12+

95%

89%

100%

FDA letter of EUA for Pfizer

Moderna

mRNA

2

18+

94.1%

100%

100%

FDA letter of EUA for Moderna

Johnson & Johnson

Viral vector adenovirus (cold virus)

1

18+

66% (U.S. 72%)

85%

100%

FDA letter of EUA for Johnson and Johnson

*Effectiveness is measured as the rate of symptoms in the placebo group minus the rate of symptoms in the vaccine group divided by the rate of symptoms in the placebo group.

Efficacy and Age

  • Pfizer
    • Ages 12-15: 100% efficacy
    • Ages 16 to 55: 96% efficacy
    • Over age 55: 94% efficacy
  • Moderna
    • Ages 18 to 654: 96% efficacy
    • Over age 65: 86% efficacy *
  • Johnson and Johnson**
    • Age 18-64: 66.1% effective
    • Age 65+: 66.2% effective

Post trials data: On March 29, 2021, the CDC reported that a study of 4000 healthcare and essential workers found that in fully vaccinated persons with either the Pfizer or Moderna vaccines, infections were reduced 90% two or more weeks after vaccination. Following a single dose of either vaccine, the participants’ risk of infection with SARS-CoV-2 was reduced by 80 percent two or more weeks after vaccination.

On April 15, 2021, the CDC reported that 5,800 so-called "breakthrough" cases of COVID-19 infection in persons fully vaccinated have been reported among the 77 million people who have been fully vaccinated nationwide.  396  of these cases required hospitalization and 74 people died. Less than 0.008% of those vaccinated became infected. These results are within the range expected, given the vaccines’ reported effectiveness, but they indicate that protection is not 100% and continued vigilance is required even after vaccination.


Read more:
CDC: CDC Real-World Study Confirms Protective Benefits of mRNA COVID-19 Vaccines
Pfizer VRBAC report
Moderna VRBAC report
Johnson & Johnson VRBAC report
CDC Grading of Recommendations, Assessment, Development and Evaluation (GRADE) review of the evidence for benefits and harms for Pfizer-BioNTech coronavirus disease 2019 (COVID-19) vaccine for persons aged 12-15

 


The basic answer is that we don’t yet know the answer to this question. Different vaccines for different diseases provide protection for lengths of time of a few months to lifetime. A variety of factors, discussed below, can limit long-term immunity.  The majority of people who were enrolled in the Pfizer, Moderna vaccine trials were followed for two months after complete (2 doses) vaccination. Johnson and Johnson reported results for people 28 days following administration of a single dose of their vaccine but continued to follow participants for twice as long. All of the vaccine manufacturers are continuing to follow participants in their studies, but many of those who received a placebo have opted out and gotten vaccinated, which will interfere with getting results from study participants. Probably the best results will come from following those who have gotten vaccinated outside the study.

Studies of immunity following natural infection have reported sufficient antibodies and t-cell memory to protect against re-infection up to 8 months post infection, however cases of re-infection while relatively rare, have been reported at shorter times than this.

The presence of mutational variants of the Coronavirus, such as the UK and South Africa variants, also complicates the issue. While some studies have indicated that the Pfizer and Moderna vaccines are very effective against the UK variant, the results with the South Africa variant are less robust and Pfizer, Moderna and the Johnson and Johnson vaccines show less effectiveness, although enough to protect against severe hospitalization and death. The amount of immunity afforded by a vaccine is limited by the possibility that new strains of the virus could develop.

Although we don’t yet have an idea how long immunity from COVID-19 vaccination lasts, the CDC recommends that those who have been vaccinated do not need to quarantine after exposure to the virus if they have been fully vaccinated in the last three months. Persons who get vaccinated are also allowed to socialize with others who have been vaccinated without wearing masks or observing social distancing.


The people in the COVID-19 vaccine trials were of the following race and ethnicity

  • Pfizer
    • 57% white
    • 26% Hispanic/Latinx
    • 10% Black
    • 4% Asian
    • 3% other racial groups
  • Moderna
    • 62% white
    • 20% Hispanic/Latinx
    • 10% Black
    • 5% Asian
    • < 3% other racial groups
  • Johnson and Johnson
    • 58.7% White  (includes Latinx)
    • 19.4% Black (includes Latinx)
    • 3.3% Asian

45.3 Hispanic or Latinx (includes White and Black)


Both the Pfizer and Moderna vaccines are a two-dose series. This means that you must receive both doses to achieve the 94% and 95% protection levels seen in the trials.

  • The Pfizer vaccine is two doses given 21 days apart.
  • The Moderna vaccine is two doses given 28 days apart.
  • The Johnson and Johnson vaccine is a single-dose vaccine.

No. The protection provided by the vaccine starts seven (7) days after the second Pfizer dose and 14 days occurs after the second Moderna. Until then, you should assume you have no proven benefit from the vaccine.

Full protection from the Johnson and is reported to be 28 days after vaccination, although efficacy was also reported to increase up to 49 days post-vaccination.

Even after you are vaccinated, all policies, protocols and public health orders related to COVID-19 will remain in place until you are notified otherwise.


The answer to this question is not known for sure at this time.  During their clinical trial 3, Pfizer reported 14 days after  receiving the first dose of vaccine, participants showed a 52% reduction in getting sick from COVID-19 compared to a 95% reduction after their second shot. A recent study of persons over age 60 found that they showed a reduction of 33% in terms of testing positive for COVID-19  14 days after receiving the first shot  (note that they measured testing positive and the Pfizer vaccine trial measured showing symptoms, plus their sample was older than the Pfizer sample and contained no one under age 60).

Moderna reported that, during their Phase 3 clinical trial, participants showed a 95% reduction in getting sick from COVID-19, but they stressed that the observation period was very limited, as was their sample size.

Both Pfizer and Moderna, as well as the FDA, have stressed the need to get both doses of the vaccine in order to receive their full benefit.


With regard to the first dose, the data are limited on how much protection you receive from only a single dose of either vaccine. Pfizer reported 52% effectiveness in reducing sickness from COVID-19 14 days after receiving the first dose of the vaccine. However, a recent study in Israel of persons over age 60 found that reduction in those who tested positive for COVID-19 14 days after their first shot was only 33% (note that they measured testing positive and the Pfizer vaccine trial measured showing symptoms, plus their sample was older than the Pfizer sample and contained no one under age 60).

Moderna has reported efficacy of from 80-95% after one dose of their vaccine, but their sample was of limited size and the duration of observation was short, so their results are considered tentative, not conclusive.

The bottom line is that after either a first or second dose of vaccine, one is not completely protected from getting COVID-19, although severe cases after vaccine are very rare. Whether one can be infected without getting sick is unknown at this time and that could make you able to transmit the virus even if you aren’t sick yourself. For all of these reasons, people are urged to continue to wear a mask and social distance, when around others who have not been vaccinated, after both their first and second doses of vaccine.


The CDC has said that fully vaccinated people*can:

  • Visit with other fully vaccinated people indoors without wearing masks or physical distancing
    • Indoor visits between fully vaccinated people who do not wear masks or physically distance from one another are likely low risk. For example, if you are fully vaccinated, it is likely a low risk for you to invite other fully vaccinated friends to dinner inside your private residence.
  • Visit with unvaccinated people from a single household who are at low risk for severe COVID-19 disease indoors without wearing masks or physical distancing
    • Indoor visits between fully vaccinated people and unvaccinated people who do not wear masks or physically distance from one another are likely low risk for the vaccinated people.
    • Therefore, the level of precautions taken should be determined by the characteristics of the unvaccinated people, who remain unprotected against COVID-19.
    • For example, fully vaccinated grandparents can visit indoors with their unvaccinated healthy daughter and her healthy children without wearing masks or physical distancing, provided none of the unvaccinated family members are at risk of severe COVID-19.
    • If any of the unvaccinated people or their household members are at increased risk of severe COVID-19, all attendees should take precautions including wearing a well-fitted mask, staying at least 6 feet away from others, and visiting outdoors or in a well-ventilated space.
  • Walk, jog or sit outdoors without masks, except in large crowds.

* People are considered fully vaccinated:

  • 2 weeks after their second dose in a 2-dose series, such as the Pfizer or Moderna vaccines, or
  • 2 weeks after a single-dose vaccine, such as Johnson & Johnson’s Janssen vaccine

If you don’t meet these requirements, you are NOT fully vaccinated. Keep taking all precautions until you are fully vaccinated.


People who are fully vaccinated* do not need to quarantine or be tested following a known exposure unless you develop symptoms

* People are considered fully vaccinated:

  • 2 weeks after their second dose in a 2-dose series, such as the Pfizer or Moderna vaccines, or
  • 2 weeks after a single-dose vaccine, such as Johnson & Johnson’s Janssen vaccine

If you don’t meet these requirements, you are NOT fully vaccinated. Keep taking all precautions until you are fully vaccinated.

For more information read: Interim Public Health Recommendations for Fully Vaccinated People


On June 17, 2021, CalOSHA approved regulations that provide the following rules for businesses within California:
 
Employers may allow vaccinated employees to work without face coverings indoors, but they must document workers’ vaccination status. That documentation process allows workers to either show written proof of vaccination, such as a CDC vaccine card, or to “self-attest” that they are vaccinated without providing documentation.
 
Workers can decline to state if they are vaccinated or not. However, they will be treated as if they are unvaccinated.
 
Unvaccinated workers must wear masks indoors, unless alone in a room or vehicle. Employers must make approved respirators, such as an N95 mask, available for unvaccinated workers if they request them.
 
No face coverings are required outdoors unless there is an outbreak.
 
If there is a COVID outbreak, masks will be mandated for all workers indoors, and outdoors if six-foot physical distancing can’t be maintained.
 
No physical distancing or barriers between workers are required, regardless of employees’ vaccination status, although employers can re-evaluate the need if an outbreak occurs. Distancing and barriers will be mandated if a “major outbreak” of 20 or more cases occurs.
 
Fully vaccinated workers with no COVID symptoms do not need to be tested or quarantined after they are exposed to the virus.
 
Employees cannot face retaliation for wearing a mask, even if they are not required to do so. signed an executive order allowing the new rules to take effect immediately.
 
The Governor of California signed an executive order allowing the new rules to take effect immediately.
 
For more information:
 
 

Fully vaccinated people may resume domestic travel and do not need to be tested before or after travel or self-quarantine after travel.

  • Fully vaccinated travelers do not need to get a SARS-CoV-2 viral test before or after domestic travel, unless testing is required by local, state, or territorial health authorities.
  • Fully vaccinated travelers do not need to self-quarantine following domestic travel.

Do not need to be tested before leaving the United States for international travel (unless required by the destination) and do not need to self-quarantine after arriving back in the United States.

  • Fully vaccinated travelers do not need to get tested before leaving the United States unless required by their destination.
  • Fully vaccinated air travelers coming to the United States from abroad, including U.S. citizens, are still required to have a negative SARS-CoV-2 viral test result or documentation of recovery from COVID-19 before they board a flight to the United States.
  • International travelers arriving in the United States are still recommended to get a SARS-CoV-2 viral test 3-5 days after travel regardless of vaccination status.
  • Fully vaccinated travelers do not need to self-quarantine in the United States following international travel.

For more information read: Interim Public Health Recommendations for Fully Vaccinated People


So far, trials indicate that COVID-19 mRNA vaccines are highly protective and generate a strong immune response. Sometimes when vaccines produce an immune response, there may be side effects that feel like the flu. This does not mean you are infected or contagious. Instead, these symptoms are a signal that your body is successfully generating an immune response to protect you from the virus. Both mRNA COVID-19 vaccines commonly cause mild-to-moderate non-infectious flu-like symptoms. Here's what we know so far about side effects:

  • Pfizer mRNA vaccine
    • Any symptoms: 59% after first dose, 70% after second dose
    • Mild to moderate symptom type: fatigue 63%, headache 55%, muscle aches 38%, chills 32%, joint pain 24%, fever 14%
    • Few grade 3 (severe) side effects: fatigue 4%, headache 2%
  • Moderna mRNA vaccine
    • Any symptoms: 87% after first dose, 90% after second dose.
    • Grade 3 (severe) side effects: pain at site: 4.6% swelling (tenderness) 0.4%, swelling (hardness) 1.7%, redness at site 1.9%,  fatigue 11%, muscle aches 10%, joint pain 6%, headache 5%, fever 1.7%
  • Johnson and Johnson vaccine
    • Any symptoms: Systemic: 55.1%
    • Any symptoms: Local: 50.2%
    • Systemic symptoms: headache, 38.9% vs. 23.7 placebo; fatigue, 38.2% vs 21.5% placebo; myalgia (muscle pain), 33.2% vs. 12.7% placebo; fever, 9% vs 0.6% placebo, nausea, 14/2% vs 9.7% placebo
    • Local symptoms: Site pain 48.1% vs 16.7% placebo; Erythema (redness), 7.3% vs 3% placebo; Swelling 5.6% vs 1.6% placebo
    • Grade 3 (serious) symptoms: Systemic: 1.8%
    • Grade 3 (serious ) symptoms: Local: 0.7%
    • All symptoms were more prevalent in those under age 60 than those over age 60.
Schedule your vaccinations when you do not have anything important planned for the next day or two, including work.

If you regularly take aspirin, acetaminophen (e.g., Tylenol) and ibuprofen (e.g., Motrin, Advil) for other medical conditions, continue to do so as directed by your physician or as needed. Otherwise, do not pre-medicate.

Taking over-the-counter medications such as acetaminophen and ibuprofen before receiving a vaccine may reduce its ability to work and blunt your immune response to the vaccine. After the vaccination, don’t hesitate to take an over-the-counter medication if you have symptoms that make you uncomfortable.


The Pfizer and Moderna trials were not designed to assess the effectiveness of a single shot. For example, everyone in the Pfizer vaccine group received two shots, 21 days apart. Even though overall data suggest that benefits may start after the first dose, we don’t know enough to make any conclusions.

Even after you are vaccinated, all policies, protocols and public health orders related to COVID-19 will remain in place until you are notified otherwise.


Try to get the second dose on time. Data on vaccine benefit was based on a fixed number of weeks between doses (three weeks between Pfizer doses; four between Moderna doses). If you are late, you should still receive the second dose.

Most vaccines have rules for how many weeks you can be late before you should start the vaccine series over. For the COVID-19 vaccines, doses will be given late until the CDC or published clinical data provide guidance on when it is too late to give a second dose.