We still don’t know how well a robust immune response protects people from SARS-CoV-2 infection. But we’ve got a further indication that vaccines can induce a strong immune response. Just prior to the holiday weekend, a Chinese team released the results of a safety trial done using a harmless virus that had been modified to carry one of the coronavirus genes. While there were a number of side effects, everyone getting the vaccine had a robust antibody response, including some antibodies that neutralized the virus.
The two reports also differ significantly in terms of their approach to generating an immune response. The earlier announcement, from a company called Moderna, involved injecting carefully packed RNAs that encode the spike protein that normally resides on the surface of the virus. The RNAs transit inside a person’s cells and induce them to produce the spike protein, thereby exposing the immune system to it.
The Chinese researchers used a very different approach to inducing immunity. In their case, they engineered the gene that encodes the spike protein into a harmless virus called Adenovirus 5. They then produced large quantities of the engineered virus and injected that into people. Even though adenoviruses are essentially unrelated to coronaviruses (they use DNA as their genetic material, rather than RNA), the cells that the engineered viruses infect will produce the coronavirus spike protein, again exposing the immune system to it.
The researchers also looked for antibodies that neutralized SARS-CoV-2, preventing it from infecting additional cells in culture. These were present in all the groups tested, with their frequency increasing with vaccine dose.
Overall, this is good news. But it’s also incomplete. Neutralizing a virus in culture is very different from neutralizing it in an actual organism. And there have been cases in the past where a vaccine has induced antibodies that make it easier for a virus to infect cells (by stabilizing the conformation in which the virus latches on to cells). So, we don’t know if the antibody production seen here will mean protection—that’s an issue for the next round of trials.
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