Sinovac COVID-19 vaccine yields neutralizing antibodies in Phase II, titers not revealed

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Sinovac’s disclosure over the weekend of early clinical data from its COVID-19 vaccine makes it the third company to demonstrate production of neutralizing antibodies in humans, behind Moderna and CanSino. Sinovac’s data come from many more patients, upping the confidence level, but without disclosure of the titers it’s hard to make even rough predictions about efficacy.

Sinovac Biotech Ltd. (NASDAQ:SVA) reported Saturday that over 90% of the 600 volunteers in the Phase II portion of a Phase I/II trial generated neutralizing antibodies (NAb) against SARS-CoV-2 at 14 days after prime-boost administration of CoronaVac. That rate is on par with the eight subjects Moderna Inc. (NASDAQ:MRNA) reported data from in May, and higher than data from 108 subjects in a study from CanSino Biologics Inc. (HKEX:6185) the same month.

Sinovac said no participants had severe adverse events in either the Phase II part of its trial or 143-subject Phase I portion.

Sinovac expects to submit the Phase II data and a Phase III study protocol to China’s National Medical Products Administration (NMPA) in the near future, and to begin Phase III testing outside China. Last week, the company announced a partnership with the Butantan Institute, which will sponsor Brazilian Phase III trials of the inactivated SARS-CoV-2 vaccine as well as support manufacturing and commercialization in Brazil.

The company, which in April became the first to report non-human primate data demonstrating a COVID-19 vaccine could confer protection, though not full sterilizing immunity, plans to publish full data from its clinical trials of CoronaVac in academic publications (see “Animal Studies Show Sinovac’s COVID-19 Vaccine Is Protective”).

The relationship between neutralizing antibodies and immunity isn’t fully understood, but beating FDA’s guidelines for neutralizing antibodies in convalescent plasma would boost confidence to some degree.

Of the three companies to report early clinical data from COVID-19 vaccines, only CanSino revealed neutralization titers, and it failed to make this standard.

CanSino reported Phase I data in a May 22 Lancet publication evaluating single immunizations with three doses of Ad5-nCoV in 108 individuals. The mean neutralization titer in the high dose group was 34 -- well below FDA’s recommended titer of 160 for convalescent plasma (see “Making Sense of Early COVID-19 Vaccine Data”).

Neutralizing antibody titers could increase with a booster vaccination, and according to ClinicalTrials.gov, CanSino is evaluating single and double immunizations with the two lower doses of the adenoviral vector COVID-19 vaccine in a randomized, controlled Phase I/II trial in Canada.

Also in May, Moderna said it had evaluated neutralizing antibody production in eight volunteers in a Phase I trial of mRNA-1273. While the company didn’t disclose the titers, it said all eight subjects -- who’d been immunized twice with either 25 μg or the company’s Phase III dose of 100 μg -- had titers at or above those generally detected in convalescent plasma (see “What Moderna’s COVID-19 Vaccine Data Does Not Say”).

Mouse data for mRNA-1273 released Thursday add a line of evidence suggesting the vaccine might be protective. Prime-boost administration of two different doses of the vaccine completely blocked SARS-CoV-2 replication in lung in all the mice; and only one mouse had signs of replication in the nasal cavity (see “Moderna Vaccine First to Produce Sterilizing Immunity in Model System”).

Sinovac made no statement about whether the neutralizing antibodies in its study were above or below convalescent plasma.

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