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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: José Eduardo Riceto Loyola Junior, M.D.[2]

Overview

On December 11, 2020, the F.D.A. authorized Pfizer's vaccine for emergency use for the prevention of COVID-19. Since then, many other vaccines have been developed, such as the ones from Pfizer, Moderna, AstraZeneca, Janssen, Sinovac, Sinopharm and Gamaleya. Efficacy, side effects and safety profiles vary dramatically between them, as they are produced using different mechanisms.

Vaccines

Pfizer/BioNtech Comirnaty - BNT162b2

- Mechanism of action: mRNA-based; PEGylated lipid nanoparticles vector.
It was approved for emergency use in December 11, 2020. It has been shown to have an efficacy of 95% at preventing COVID-19 in persons 16 years of age or older. Its protection against severe COVID-19 was shown to be of approximately 97%. New analysis showed that after six months its efficacy fell to 84%, which is not known if this is due to the vaccine and immune system themselves of if the emergence of variants are affecting the efficacy of the vaccine. As for side effects, the vaccine has been reported to cause mostly mild symptoms such as myalgia, headaches and soreness in the location where it was applied. Allergic reactions have also been reported in a few patients, and they all recovered quickly after an epinephrine shot. It has been theorized that the allergic reactions were mediated by the PEGylated lipid nanoparticles in which the mRNA is stabilized. It has also been reported to be associated with myocarditis and pericarditis, especially in young men, but the cases reported so far were mild and recovered. Efficacy against the Delta variant is of about 88%, while against the alpha variant, in the same study, it was about 93.7%.

• Dose regimen:
  • Application of 2 doses, spaced by 21 days between shots.
  • A third shot was recently recommended by the F.D.A. for immunocompromised patients.
• Must be kept at very low temperatures to preserve its substrate, between -80 and -60C. Undiluted vials must be kept in the refrigerator at 2 to 8C for up to 5 days, and it must not be kept at room temperature for more than 30 minutes during its administration.

Moderna - mRNA-1273

- Mechanism of action: mRNA-based; lipid nanoparticles vector.
Approved for emergency use by the F.D.A. on December 18, 2020. The clinical trials produced by Moderna showed that its vaccine has an efficacy of 90% against symptomatic COVID-19 and 95% efficacy against severe disease after six months. Side-effects include: arthralgia, myalgia, fever, chills, headache, nausea or induration/pain at application site.. No allergic reaction has been described with Moderna's vaccine so far, in comparison to Pfizer's, but it has been associated with a bigger occurrence of mild side-effects in comparison to the latter. It has also been associated with pericarditis and myocarditis, but these were mild cases, as was the case with the Pfizer's one.

• Dose regimen:
  • Application of 2 doses, spaced by 28 days between shots.
  • A third shot was recently recommended by the F.D.A. for immunocompromised patients.
• Must be kept in temperatures of -25 to -15C in order to conserve its substrate. Vials can be kept in the refrigerator at 2 to 8C for up to 30 days. After first application, vial must be discarded after 6 hours.

Janssen - Ad26.COV2.S

- Mechanism of action: adenoviral vector.
Authorized for emergency use by the F.D.A. at February 27, 2021. According to the CDC, its efficacy has been estimated to be at approximately 67% in preventing moderate to severe COVID-19 after at least 14 days after vaccination and 66% after at least 28 days. Regarding only severe cases, its efficacy has been of approximately 77% at least 14 days after vaccination ad 85% at least 28 days after vaccination. According to the CDC, the most common side effects are: headache, fatigue, nausea, pain at the injection site and muscle aches, lasting 1-2 days after injection. It was also associated with Guillain-Barré syndrome and also with vaccine-induced thrombocytopenia and thrombosis.

• Dose regimen: single dose
• Must be kept at temperatures between 2 to 8C in order to conserve its substrate. Unpunctured vials may be kept between 9 to 25C for up to 12 hours.

AstraZeneca/Oxford - ChAdOx1 nCoV-19

- Mechanism of action: adenoviral vector.
It has been associated with vaccine-induced immune thrombocytopenia and thrombosis. Efficacy against Delta variant is 67% after two doses, in comparison to 74,5% against the alpha variant.. It was also associated with Guillain-Barré syndrome and also with vaccine-induced thrombocytopenia and thrombosis.

Vaccine-induced Thrombocytopenia and Thrombosis

Diagnostic criteria - must have all four:

• COVID vaccine 42 days previously
• Any venous or arterial thrombosis (often cerebral or abdominal)
• Thrombocytopenia
• Positive PF4 heparin-induced thrombocytopenia ELISA

Signs and symptoms
Headache, visual abnormalities, nausea and vomiting, back pain, leg pain and swelling, abdominal pain, shortness of breath, petechiae, bruising or bleeding.
Work-up
Imaging to screen for thrombosis + cell blood count to assess thrombocytopenia, PF4-ELISA (HIT assay), fibrinogen and D-dimer.
Treatment
If confirmed: start intravenous immune immunoglobin and nonheparin anticoagulation.

SinoVac - CoronaVac

- Mechanism of action: inactivated virion.
Most widely used vaccine in the world against SARS-CoV2, according to Airfinity. A Chilean cohort study. stated that its efficacy is at approximately 65.9% for preventing COVID-19 in fully immunized patients, 87.5% for prevention of hospitalization and 90.3% for prevention of ICU admission. Its efficacy for prevention of COVID-19-related death was estimated at 86.3%.. A study performed by Butantan Institute reported an efficacy for prevention of COVID-19 infection of 50.4%, 77% for prevention of mild disease and 100% efficacy on prevention of severe disease, but the numbers were not statistically significant. Despite these results, some deaths of fully vaccinated patients in the country have been reported, specially for its elderly population, which was the one that was vaccinated with CoronaVac. A non-peer reviewed study reported that it had a real-world efficacy of only 42% in the elderly population and that its efficacy decreases with age. In June, due to a reduction in its previous efficacy estimate, Chile started to give to its population older than 55 years old that were fully vaccinated with Coronavac a shot of AstraZeneca's vaccine.

• Dose regimen:
  • Application of two doses spaced by at least 4 weeks.