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How safe is the vaccine?
The vaccine safety trials showed that, outside of minor side effects that are common with other vaccines, all three currently approved vaccines, Moderna, Pfizer and Johnson & Johnson, are extremely safe. However, it has been reported that individuals with a history of severe allergies (i.e. anaphylactic shock) should take precaution when getting the vaccine, although these incidences are extremely rare. Additionally, now that the vaccines have been given to hundreds of millions of people worldwide since the beginning of this year, adverse reactions have been found to be extremely rare.
Two reported rare side effects linked to vaccination include blood clotting in young women (Johnson and Johnson) and myocarditis in young men (Pfizer and Moderna), but the incidences are extremely low (0.5-1 cases per million doses for blood clotting and 10-75 cases per million doses in young men for myocarditis). However, it has been established that the risks of severe health complications are much higher for members of these age groups that contract COVID-19.
The Pfizer, Moderna and Johnson and Johnson vaccines do not contain the virus, and the contents of the vaccine are similar to normal chemicals made by our cells. Therefore, the vaccine is considered extremely safe. For example, the lipid envelopes enclosing the mRNA vaccines (i.e. Pfizer and Moderna) contents are simple phospholipids just like your normal cell’s membranes. The RNA is released inside your cells, where it is used to make a protein in the same way all your proteins are made. The Johnson and Johnson vaccine utilizes DNA that is packaged in a harmless adenovirus vector to encode viral spike protein. For all vaccines, none of the vaccine components can change your DNA.
Per CDC recommendation, pregnant women should consult their doctor, as there have been no clinical trials looking at either Moderna or Pfizer vaccine safety in pregnant women. The World Health Organization currently advises that pregnant women not take either mRNA vaccines until further clinal trials are done. Although no data has been done looking at pregnant women with the Johnson and Johnson COVID-19 vaccine, other viral vector vaccines from Johnson and Johnson have been examined in pregnant women. In these studies, there was no complications during the pregnancy.
Will I get sick from the COVID-19 vaccine?
The Moderna and Pfizer mRNA vaccines do not contain any viral material, therefore you will not be infected with the virus. The second dose of these COVID-19 vaccines does generate a very strong immune response that may cause you to have mild soreness and temporary lethargy. In clinical trials, people typically demonstrated minor side effects following the second-dose of the vaccine, including low-grade fever, soreness at the infection site, fatigue, chills and nausea/vomiting. These side effects typically last 1-2 days after vaccination. The Johnson and Johnson vaccine contains an adenovirus vector that is unable to replicate, meaning that you cannot become infected with a virus. However, similar to the mRNA vaccines, there can be side effects that include low-grade fever, soreness at the infection site, fatigue, chills and nausea/vomiting. In very rare cases for all three vaccines, people with a history of severe allergic reactions have had more serious adverse effects. Therefore, people in this group should consult their doctor prior to receiving a vaccine.
Has mRNA technology been around long enough for it to be safe in humans?
Although the Pfizer and Modera COVID-19 vaccines are the first mRNA vaccines approved by the FDA for human use, the use of mRNA to generate an immune response in animal models dates back to 1990, and has been researched extensively. Lipid nanoparticles utilized in the vaccine also have been researched in animal models for several years. Prior to the COVID-19 pandemic, there were already mRNA vaccines for other viruses in Phase I and II trials.
Has viral vector technology been around long enough to use it in humans?
Although the Johnson and Johnson COVID-19 vaccine is the first viral vector vaccine approved by the FDA for human use, the use of viral vectors in animal models dates back to the 1970s, and has been researched extensively. Prior to the COVID-19 pandemic, there have been several viral vector vaccines extensively researched and in clinical trials. Furthermore, viral vector vaccines have been used against Ebola during recent outbreaks in Africa.
To get the vaccine approved so quickly were there short cuts taken during the approval process?
There were no short cuts taken for approval of the COVID-19 vaccine. In order to be approved by the FDA, each drug must undergo three Clinical Trial Phases. These phases are typically done in sequential order, with increasing numbers of test subjects in each, with each trial typically taking months to years to complete, depending on the drug examined. However, for the COVID-19 vaccines, some of the phases “stacked”, allowing for expedition of the normal approval process. However, the same number of subjects were enrolled for the trials that are normally required, and the same safety and effectiveness standards were applied when compared to other approved vaccines. Moderna had a Phase 1 trial with a few dozen people, a Phase 2 with a few hundred people and then Phase 3 with 30,000 people. Pfizer had a Phase 1 trial with a few dozen people, and did a combined their Phase 2/3 trial with ~45,000 people. Johnson & Johnson had a combined Phase 1/2 trial with ~800 people, and then a Phase III trial with ~40,000 people in it. The results from all of these trials showed high efficacy against symptomatic infections and had a very high safety profile for each of the vaccines. The clinical trial results were analyzed and reviewed after a few months vs. the normal 6 month timeline. Which is why they are under emergency use authorization. However, both Pfizer and Moderna have applied for Full FDA Approval, which is expected to go through shortly.
How long does it take to generate a protective immune response to COVID-19 after the vaccine?
Full protection is acquired two weeks after the second dose of both of the mRNA vaccines (Pfizer or Moderna). Although, the first dose does offer some protection 14 days after vaccination (~80% for the Moderna), how long that protection lasts is currently unknown. For the Johnson and Johnson vaccine only requires one dose. Although It takes 14 days for some protection to develop, after 28 days vaccine recipients showed 100 percent protection against hospitalizations and death. It typically takes the body 14 days to begin to generate an effective antibody response. Therefore, if you contract SARS-CoV-2 during that period, you may not be fully protected from COVID-19.
If I get my first dose from Moderna, can I use Pfizer for the second dose or vice versa?
Studies about “mix and match” vaccine delivery are underway, but for now this is not recommended. Although both vaccines use similar materials, and the same mechanism to provide protection (see next question), no definitive studies have been undertaken to determine the safety of using one of each vaccine for doses. Therefore, if you are vaccinated with either Moderna or the Pfizer vaccine on the first dose, you are required to be vaccinated with the same on the second dose either 28 days or 21 days post vaccination, respectively.
How does the mRNA vaccine (Pfizer or Moderna) protect me from COVID-19?
The mRNA vaccine contains a lipid nanoparticle that helps protect the mRNA. This lipid nanoparticle fuses with the lipids that surround our cells, releasing a mRNA inside of our cells. mRNA is then recognized by our normal cellular machinery responsible for creating our body’s proteins and make the SARS-CoV-2 Spike protein. The Spike protein is then released from the cell, where it is picked up by immune cells and used to generate an immune response. This results in the generation of SARS-CoV-2 Spike protein-specific neutralizing antibodies, as well SARS-CoV-2-specific CD8 T cells. This process takes up to 14 days to occur. However, upon infection with the virus, the antibodies and CD8 T cells are able to interact with the virus very quickly after infection to produce a much stronger and efficient immune response, protecting the vaccinated individual from the severe symptoms of COVID-19.
How does the Johnson and Johnson viral vector vaccine protect me from COVID-19?
The Johnson and Johnson vaccine contains a modified Adenovirus vector (Ad26) that contains DNA. This Adenovirus vector is able to interact with our cells, which then allows for it to release the DNA that encodes for the SARS-CoV-2 Spike protein inside of our cells. It is important to note that although the adenovirus is able to interact with our cells, the vector has been stripped of its ability to replicate, therefore is unable to establish an infection. The DNA is then recognized by our normal cellular machinery to create RNA, which then is used to make the SARS-CoV-2 Spike protein. The Spike protein is then released from the cell, where it is picked up by immune cells and used to generate an immune response. This results in the generation of SARS-CoV-2 Spike protein-specific neutralizing antibodies, as well SARS-CoV-2-specific CD8 T cells. This process takes up to 14 days to occur. However, upon infection with the virus, the antibodies and CD8 T cells are able to interact with the virus very quickly after infection to produce a much stronger and efficient immune response, protecting the vaccinated individual from the severe symptoms of COVID-19.
Can I still spread COVID after vaccination?
Early data has demonstrated that the vaccines are highly effective against transmission. Furthermore, those that do show symptoms shed less virus and shed virus for less time, meaning that chances of vaccinated individuals transmitting the virus is much lower. Therefore, the CDC recommendation is that if you are vaccinated, then you are not required to wear a mask. However, the vaccine is not 100% effective at blocking transmission, therefore if you live or work with those who are high risk, it may be your choice to wear a mask for extra protection. The vaccine’s effectiveness at preventing spread of the Delta variant is still under study, but there is some evidence of mildly reduced efficacy against symptomatic disease among older patients, although protection against severe disease is still appears to remain very strong.
How long will my protection last from the vaccine?
This is currently still under study, but it is early data has pointed to protection against severe disease (i.e., requiring hospitalization) lasting a long time (at least 1-2 years). However, protection against all symptoms, including mild symptoms, is still being studied and likely will not last as long. Pfizer has recently indicated they have evidence of waning protection against symptomatic infection. However, it is still is highly effective against hospitalizations and deaths.
If I have already had COVID-19, do I need to get vaccinated?
Yes. Even if you already have had COVID, you should still get vaccinated. At this point, it is unclear how long natural immunity lasts to COVID-19, and there have been several documented cases of reinfection. Additionally, several studies have suggested that vaccination may confer stronger immunity than natural infection. Therefore, it is recommended that even if you have been infected, you should get vaccinated. There is no recommended time period after infection that you need to wait before vaccination. If you were treated with passive antibodies (i. e. convalescent plasma), you must wait 90 days after treatment before getting vaccinated. If you do not, the vaccine may not be effective.
Will the COVID-19 vaccine be more effective than the seasonal influenza vaccine?
Yes, the COVID-19 vaccine is expected be more effective than the seasonal influenza vaccine.
All viruses mutate, allowing for the potential of the virus to evade the body’s previous immune response, even those primed by a vaccine. Influenza viruses have a higher mutation rate than SARS-CoV-2 and are also more prone to genetic rearrangements that allow for it to generate strains that evade the body’s immune response. This heightened ability to mutate results in extremely variable vaccine effectiveness of the seasonal influenza vaccine. While we don’t yet know the long-term effectiveness of the current COVID-19 vaccines, there is hope that they will behave more like vaccines designed for viral diseases such as smallpox, measles, polio, mumps, rubella by providing longer-lasting protection.
Will the Pfizer and Moderna vaccines work against new mutant strains of SARS-CoV-2?
Studies for the Pfizer, Moderna and Johnson and Johnson vaccines have shown very little to no significant decrease in protection against symptomatic disease to the Alpha strain (B.1.1.7, UK). For both the Pfizer vaccine and the Johnson and Johnson vaccine, there is a slight decrease in protection against symptomatic infection to the Beta strain (B.1.351, South African). However, protection against severe disease (i.e. hospitalizations and death) is still very high for both vaccines. Real world data for the Moderna against both the Beta strain and the Gamma strain (P.1, Brazil) is limited, and the Pfizer vaccine against the Gamma strain. However, lab studies suggest a decrease in neutralizing antibody protection, but both vaccines are still expected to be largely protective against symptomatic disease, and to be highly protective severe disease. Johnson and Johnson vaccine efficacy against the Gamma strain showed a slight decrease, but still retained very high protection against severe disease.
Data on the newly emerged Delta variant (B.188.8.131.52, India) is still limited and conflicting. For the Pfizer vaccine, early results from the UK and Israel indicated a slight decrease in protection against mild symptomatic infection after two doses. However, protection against severe infection is still very high (~95%). There is limited data for Moderna, although a study has indicated that just one dose of Moderna is highly effective against severe disease. Lab studies for both Moderna and Johnson and Johnson have suggested a modest decrease in neutralizing antibody protection, but are still expected to be highly effective against the Delta variant. The Delta variant has another version (Delta Plus) that has obtained an additional mutation seen in the Beta variant. This mutation has been associated with evasion against antibodies, but there is no data at this point on the impact of vaccine efficacy against this variant.
While it is possible for viruses to evolve complete resistance to vaccines, there is currently no evidence suggesting that this has occurred. However, the sooner that widespread vaccination has occurred and the number of infected people spreading the virus is reduced, the fewer opportunities the virus will have to evolve resistance to current vaccines. In other words, “If it can’t replicate, it can’t mutate”. For these and others reasons, rapid vaccination of the as much of the population as possible is critical to stopping the COVID-19 pandemic.
Which vaccine should I get if I am offered?
The short answer is: Any of the vaccines. Although the Johnson and Johnson vaccine demonstrated decreased efficacy in preventing any COVID-19 symptoms compared to the Moderna and Pfizer vaccines, all three vaccines demonstrated 95-100 percent efficacy in preventing COVID-19 hospitalizations and deaths.
Are vaccines safe for children aged 12-18?
Children 12 and up can currently receive the Pfizer vaccine and both research trials and clinical data have confirmed that vaccinated children are not adversely affected, except in extremely rare cases.As noted above in Question 1, vaccinated young men are at slightly higher risk for myocarditis, but the risk factors are dwarfed by health risks posed by COVID-19 itself. For instance, there are many reports of long term health complications in children with COVID-19, including thousands of cases of Multisystem Inflammatory Syndrome in the USA.
Although infected children from ages 12-18 years are less likely to develop severe disease than adults, several studies have shown that children shed virus at higher levels than adults, demonstrating that children are able to spread the virus. Additionally, the Delta variant has been reported by several countries to be impacting younger populations, including children, at a higher rate that previous strains. With this, more and more cases of infected children becoming extremely ill, have demonstrated that COVID-19 poses a serious threat to unvaccinated children.
Additionally, there is no evidence of vaccines impacting women’s fertility. Although pregnant women were excluded in the vaccine trials, there were several pregnancies that occurred during the different vaccine trials. These pregnancies occurred at the same rate between the vaccine and placebo group, and there have been no differences side effects during these pregnancies between the two groups.
Phase II/III trials for children aged 6 months to 11 years are currently underway for both Pfizer and Moderna, and initial results are expected for the 5 to 11 tear-old cohort in the late summer or early fall of 2021
For more information about the Moderna Vaccine, including its contents and safety profile and its contents, please visit the Moderna website, the FDA website and/or the CDC website:
For considerations about using different vaccines, or if you have had COVID-19 before, please visit the following link.
For more information on mRNA and Lipid Nanoparticles technology, please see the following articles:
How the Moderna vaccine works:
How the nine coronavirus vaccines work:
More vaccine questions answered:
Information on the Johnson and Johnson Viral Vector Vaccine