The day after data appeared from the vaccine maker Sinovac showed complete protection of rhesus monkeys by their vaccine candidate (whole inactivated SARS-CoV-2 virus particles), scientists from the Jenner Institute in Oxford issued a press release announcing that their vaccine (an adenovirus vector based vaccine that carried the SARS-CoV-2 spike protein) worked to protect rhesus monkeys and that they were moving forward with large scale human safety trials. At the time, the substantiating data was not available. Now it is, in the form of a May 13 BioRxiv preprint. Does the data support the claim?
Not really. All of the vaccinated monkeys treated with the Oxford vaccine became infected when challenged, as judged by recovery of virus genomic RNA from nasal secretions. There was no difference in the amount of viral RNA detected from this site in the vaccinated monkeys as compared to the unvaccinated animals. Which is to say, all vaccinated animals were infected. This observation is in marked contrast to the results reported from Sinovac trial. At the highest dose studied, no virus was recovered from vaccinated monkeys from the throat, lung, or rectum of the vaccinated animals.
There is a second troubling result of the Oxford paper. The titer of neutralizing antibody, as judged by inhibition of virus replication by successive serum dilutions as reported is extremely low. Typically, neutralizing antibodies in effective vaccines can be diluted by more than a thousand fold and retain activity. In these experiments the serum could be diluted only by 4 to 40 fold before neutralizing activity was lost. Again, by contrast the titer of neutralizing antibodies in the serum of those vaccinated with whole inactivated SARS-CoV-2 was high.
What then is the argument for pressing forward with the adenovirus vector SARS-CoV-2 spike protein vaccine?
The authors present evidence to the effect that, although the vaccine did not protect the animals from infection, it did moderate the disease.
