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COVID-19 Vaccines
Of the 3 available vaccines, the Pfizer-BioNTech COVID-19 (BNT162b2) vaccine is approved for individuals aged ≥ 16 years, while the Moderna COVID-19 (mRNA-1273) and the Johnson and Johnson/Jannsen COVID-19 (JNJ-78436735) vaccines are approved for individuals aged ≥ 18 years, though the latter vaccine has been temporarily suspended.1,3-5 The EUAs were released following the disclosure of the results of 3 phase 3 clinical trials and several phase 1 and 2 clinical trials.9-16
The BNT162b2 vaccine from Pfizer-BioNTech encodes the SARS-CoV-2 full-length spike protein (S) in prefusion conformation locked by the mutation in 2 prolines.9 Differently from the BNT162b2 vaccine, the BNT162b1 vaccine encodes a secreted trimerized SARS-CoV-2 receptor–binding domain. The S-glycoprotein is required for viral entry, as implicated in host cell attachment, and is the target of the neutralizing antibodies. In a phase 1 clinical study on 195 volunteers treated with BNT162b1 (10 mg, 20 mg, 30 mg, or 100 mg doses) or BNT162b2 (10 mg, 20 mg, or 30 mg doses) vaccines or placebo 21 days apart, both the binding and neutralizing antibody response was found to be age and “somewhat” dose dependent.9
Higher neutralization titers were measured at day 28 and 35 (7 and 14 days after the second dose, respectively) and compared with titers of persons who recovered from a COVID-19 infection.9 Serum neutralization was measured using a fluorescence-based high-throughput neutralization assay, while binding activity was assessed using the receptor-binding domain (RBD)–binding or S1-binding IgG direct Luminex immunoassays.
The overall reactogenicity/immunogenicity profile of BNT162b2 administered twice (30 mg each time) led to its selection for the phase 3 clinical trial.9,10 In a large phase 3 clinical trial on 43,458 participants, the BNT162b2 vaccine given at 30 mg doses 21 days apart conferred 95% clinical efficacy in reducing the likelihood of being affected by symptomatic COVID-19.10 No safety concerns to stop the trial were identified, though related severe and life-threatening events were reported in 0.3% and 0.1% of the volunteers, respectively. We note that these incidence rates were the same for the treated and the placebo group.
The mRNA-1273 vaccine from Moderna also encodes the SARS-CoV-2 S-glycoprotein. In a dose escalation phase 1 trial of 45 participants aged between 18 and 55 years (25 mg, 100 mg or 250 mg, given at days 1 and 29) and 40 participants aged ≥ 57 years (25 mg and 100 mg, given at days 1 and 29), a dose-dependent effect was observed for both binding (receptor-binding domain and S-2p IgG on enzyme-linked immunosorbent assay [ELISA])and neutralizing antibodies (SARS-CoV-2 nanoluciferase high-throughput neutralization assay, focus reduction neutralization test mNeonGreen and SARS-CoV-2 plaque-reduction neutralization testing assay) development.11,12 The geometric mean of both binding and neutralizing antibodies declined over time but persisted high as late as 119 days after the first burst of 100 mg dose.13 The same dose of the vaccine also elicited a strong T helper-1 response with little T helper-2 response across all ages.11 The strength of the memory cellular response remains to be defined and is the subject of ongoing investigations. In a large phase 3 clinical trial with 30,420 participants, the Moderna COVID-19 mRNA-1273 vaccine, given 28 days apart at the dose of 100 mg, met 94.1% clinical efficacy in reducing the likelihood of being affected by symptomatic COVID-19.14
Less than 0.1% of volunteers in both groups withdrew from the trial due to adverse effects (AEs); 0.5% in the placebo group and 0.3% in the treated group had AEs after the first dose, which precluded receiving the second dose.14
The Johnson and Johnson/Jannsen JNJ-78436735 vaccine is based upon a recombinant, replication-incompetent adenovirus serotype 26 (Ad26) vector, which encodes the full-length, stabilized S-glycoprotein of SARS-CoV-2. The currently reported results of the phase 1 and 2 clinical study indicated that 805 volunteers (402 participants between ages 18 and 55 years and 403 individuals aged ≥ 65 years) were randomized to receive a single or double dose of either 5 x 1010 viral particles per 0.5 mL (low dose) or 1 x 1011 viral particles per 0.5 mL (high dose), each compared with a placebo group. Incidence of seroconversion to binding antibodies against the full-length stabilized S-glycoprotein, as measured by ELISA, showed ≥ 96% seroconversion by day 29 after the first dose. The incidence of seroconversion to neutralizing antibodies was ≥ 90% as early as early as 29 days after the first of either dose. In this study, neutralization activity was measured using the wild-type virus microneutralization assay based on the Victoria/1/2020/ SARS-CoV-2 strain.15 We note that the data related to this study have been partially reported and additional information will be available when each participant will have received the second dose.
In a large phase 3 clinical trial with 40,000 participants aged between 18 and 100 years, the Johnson and Johnson/Jannsen JNJ-78436735 vaccine, given as single dose of 5 x 1010 viral particles per 0.5 mL, met 65.5% clinical efficacy in the likelihood of being affected by symptomatic COVID-19 ≥ 28 days postimmunization.16 In this study, the vaccine efficacy was found to have a geographic distribution with highest efficacy in the US (74.4%), followed by Latin America (64.7%) where Brazil showed a predominance of the P2 COVID-19 lineage (64.7%), and Africa (52%) where the B.1.351 lineage was most frequent (94.5%). The vaccine also proved to be effective in reducing the likelihood of asymptomatic seroconversion, as measured by the level of a non-S protein, eg, 0.7% of positive cases in the vaccine group vs 2.8% in the placebo group. Immunological data indicated that the vaccine response was mainly driven by T-helper 1 lymphocytes. As of April 13, 2021 the FDA has recommend suspending the administration of the Johnson and Johnson/Janssen vaccine due to the occurrence of severe blood clots reported in a 6 subjects out of ~6.8 millions administered doses.1
It is noteworthy to highlight that all vaccines reduced the likelihood of hospitalizations and deaths due to COVID-19.
As of April 17, 2021, the CDC reports that more than 130 million (40%) Americans, nearly 1/3 of the population, have received at least 1 dose of any of the 3 available vaccines, including 4.6 million at the VHA.17 Using the Vaccine Adverse Event Reporting System and v-safe, the US is conducting what has been defined the most “intense and comprehensive safety monitoring in the US history.”18 Thus far, data affirm the overall safety of the available vaccines against COVID-19. Individuals should not receive the COVID-19 vaccines if they have had a severe allergic reaction to any ingredient in the vaccine or a severe allergic reaction to a prior dose of the vaccine. Additionally, individuals who have received convalescent plasma should wait 90 days before getting the COVID-19 vaccine.