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Daily chart: comparing monkey data from COVID-19 vaccines

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Three protocol differences in particular make it difficult to compare non-human primate data from the SinoVac and University of Oxford COVID-19 vaccines.

Sinovac Biotech Ltd. (NASDAQ:SVA) delivered three doses of its PiCoVacc vaccine, whereas Oxford delivered a single dose of ChAdOx1 nCoV-19, making it hard to know whether the strength of the vaccine or the number of doses was the reason SinoVac’s vaccine yielded higher titers of neutralizing antibodies faster.

The second major difference was the viral challenge given to test protection. SinoVac delivered its challenge intratracheally while Oxford split its dose up, delivering the virus via four routes. Although Oxford delivered more than double the total amount of SARS-CoV-2 as SinoVac, far less of the virus was delivered to the lungs via the trachea. In its preprint, Oxford states that the multiroute method likely does not reflect human exposure.

The studies also tested different outcomes.

Both vaccines prevented interstitial pneumonia in all vaccinated animals, whereas all of SinoVac's control animals developed pneumonia and two out of three Oxford controls. The findings suggest both vaccines achieved some level of protection.

However, Oxford’s vaccine did not reduce viral load seven days after challenge when tested via nose swab. Theoretically, the monkeys could still transmit the infection (see "Preclinical Data Suggest Oxford Vaccine Could Reduce COVID-19 Symptoms, Won’t Provide Herd Immunity").

Three of the six vaccinated monkeys in the Oxford study also developed an elevated respiratory rate, a sign of clinical disease.

SinoVac did not test viral load with a nose swab, nor did it measure respiratory rate.

In clinical trials, Oxford is delivering double the dose it gave the monkeys followed by a booster dose at half the concentration. SinoVac is delivering two doses at either high (1,200 SU/0.5 mL) or medium (600SU/0.5ml) concentration. SinoVac did not disclose the details of dosing in its non-human primate publication.

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