This evidence synthesis has been compiled by members of the CITF Secretariat and does not necessarily represent the views of all CITF members.

By Jeanie Quach

Robust immune responses in previously infected SARS-CoV-2 individuals after a single vaccine dose

This study reported that individuals with pre-existing immunity have 10-20 times higher SARS-CoV-2  antibody titers than naïve individuals after the first dose. These antibody titers remain elevated for individuals with pre-existing immunity post second dose. It was further shown that vaccine recipients with pre-existing immunity experienced higher frequencies of systemic side effects  than  naïve  recipients.  Further long-term follow-up of the immune responses of these individuals will inform if a single dose is sufficient for full immunity.

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Another vaccine can now join the fight to reduce COVID-19. Russia approved the Sputnik-V vaccine back in August 2020 after the results of the phase 1&2 clinical trials. They recently published their phase 3 clinical trial results in The Lancet. This vaccine has been approved by many Latin American and African countries.

A heterologous recombinant adenovirus-based COVID-19 vaccine appears safe and effective

The vaccine is similar to the vaccine developed by AstraZeneca and Oxford University, except that it uses a slightly different adenovirus for the second booster shot. The Gam-COVID-Vac is  a  combined  vector  vaccine based on rAd type 26 (rAd26) and rAd type 5 (rAd5) and they were administered separately with a 21-day interval. The randomized, placebo-controlled phase 2 trials, done at 25 hospitals and clinics in Moscow, Russia, included 21,977 people. There were robust cellular immune responses in all vaccinated participants by day 28 compared to day 1. The vaccine was well tolerated, with mostly  grade 1 adverse  events including flu-like illness, injection site reactions, headache, and asthenia. Some limitations of this analysis include small sample sizes within age strata, only symptomatic cases of COVID-19  were included,  and a short follow-up time for assessing immunity. This interim analysis showed 91.6% efficacy against COVID-19 from day 21 after the first dose.

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The Pfizer BioNTech vaccine protects rhesus macaques from SARS-CoV-2

Both mice and rhesus macaques were immunized with the Pfizer BioNTech vaccine, with RNAs encoding variations of the SARS-CoV-2 spike protein, BNT162b1 or BNT162b2, to assess immunogenicity. High  levels  of SARS-CoV-2 antibodies were observed in both models. Furthermore, BNT162b2 induced a strong T-cell response. Interestingly, following administration of Dose 2 to macaques, neutralizing  titres  elicited  by  either vaccine increased greater than that of a human SARS-CoV-2 convalescent  serum  panel.  These  results further support the selection of BNT162b2 for clinical testing with a larger number of participants.

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