Part 3


Vaccine development occurs in three different clinical stages. Phase 1 trials are designed to test vaccine safety. Phase II trials expand the safety profile and immune response assessment in a larger number of study participants. Phase III trials determine the effectiveness in preventing the disease. Vaccine development for the SARS-CoV-1 and Middle East respiratory syndrome coronavirus (MERS-CoV) paved the way for the rapid development of COVID-19 vaccines.

COVID-19 vaccines are developed using several different platforms. The traditional approach can include inactivated virus or live attenuated virus, which have been used for the inactivated influenza vaccine and measles vaccine.

Inactivated vaccine and live attenuated vaccines for COVID-19 are being developed and studied. RNA vaccines were the first vaccines for SAR-CoV-2 to be produced. In the United States, the COVID-19 vaccine (Pfizer-BioNTech) approved by the FDA and COVID-19 mRNA vaccine (Moderna) and adenoviral vector (Janssen or Johnson & Johnson) have been granted emergency use authorization (EUA) for prevention of COVID-19.

Pfizer (BNT162b2) COVID-19 vaccine is indicated for individuals aged 12 years or older. Moderna (mRNA-1273) is indicated for individuals aged 18 years or older. Janssen (Ad26.COVD2.S) is indicated for individuals aged 18 years or older. The choice between these COVID-19 vaccines is based on availability. There are no direct head to head studies comparing the three different vaccines effectiveness. There are possible rare adverse events in the vaccines. Janssen or Johnson & Johnson can be associated with blood clots with low platelets and possibly Guillain-Barre syndrome. The mRNA vaccines can be associated with myocarditis. The risks of these adverse events are extremely small, and the benefits of the vaccine outweigh the risks.


Pfizer (BNT162b2) vaccine is given in two intramuscular doses, given three weeks (21 days) apart. Moderna (mRNA-1273) is given in two intramuscular doses, given one month (28 days) apart. Janssen (Ad26.COVD2.S) or Johnson & Johnson vaccine is given in one intramuscular dose.

If someone has a history of SARS-CoV-2 infection, it is suggested that these individuals receive a COVID-19 vaccine. These individuals should have recovered from acute infection and meet criteria for stopping of isolation precautions before receiving the vaccine. It is possible that they may be more likely to experience local and systemic adverse effects than those who are COVID-19 negative prior to receiving the vaccine. Please consult your healthcare provider for your individual condition and recommendations.

If someone has an immunocompromised condition, it is suggested they undergo COVID-19 vaccination. In addition, it is recommended that these individuals receive a third dose at least 28 days after the second dose. An immunocompromised condition can include cancer, chemotherapy for cancer, solid organ transplant, HIV infection and use of immunosuppressive medications. Please consult your healthcare provider for your individual condition and recommendations.

Pfizer-BioNTech (BNT162b2) is a mRNA vaccine. In phase III trial, the vaccine has a 95% efficacy in preventing symptomatic COVID-19 at or after day 7 following the second dose. In phase III trial, the Moderna (mRNA-1273) has a 94.1% efficacy in preventing symptomatic COVID-19 at or after 14 days following the second dose. Janssen/Johnson & Johnson (Ad26.COV2.S) is an adenovirus vector vaccine. In phase III trial, the vaccine has a 66.9% efficacy in preventing moderate to severe/critical COVID-19 starting at or after 14 days following vaccination.


Janssen or Johnson & Johnson (Ad26.COV2.S) vaccine has been associated with an extremely small risk of unusual type of blood clot events. A similar risk has not been identified with the mRNA vaccine. The blood clots occur at unusual sites. Myocarditis and pericarditis have been reported with the mRNA vaccines including Pfizer (BNT162b2) and Moderna (mRNA-1273) vaccines. A similar risk has not been reported with the Janssen or Johnson & Johnson (Ad26.COV2.S) vaccine. Most reported myocarditis cases were mild and responded well to medical treatment. Guillain-Barre syndrome has been observed in the Janssen or Johnson & Johnson (Ad26.COV2.S) vaccine but has not been observed with the mRNA COVID-19 vaccines.

Breakthrough infections after vaccinations can occur because no COVID-19 vaccine is 100 percent effective. However, the breakthrough infection after vaccination occurs less and a high proportion may be asymptomatic. Because of the possible waning immunity and decreased efficacy against variants, the role of booster vaccinations is still being investigated. In the United States, there are announced plans to administer booster vaccinations. The WHO (World Health Organization) notes that the evidence and use of booster vaccination remains inconclusive. The United States has several systems to assess safety which are both passive and active. This can include the Vaccine Adverse Event Reporting System (VAERS) where providers, parents, and patients report adverse events. The Vaccine Safety Datalink (VSD) is a joint project between the United States Centers for Disease Control and Prevention (CDC’s) Immunization Safety Office and eight health care organizations to actively monitor safety of the vaccines. The Clinical Immunization Safety Assessment project (CISA) is a national network of vaccine safety experts from the CDC’s Immunizations Safety Office, seven academic medical research centers, and subject matter experts, and it provides a comprehensive vaccine safety public health service to the nation.


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