Day 5: What happened with the Merck vaccine?

While the prospect of a therapeutic vaccine might be looking up, there still isn’t much good news on the prospect for a preventative vaccine. Merck announced last week that their new vaccine candidate may have actually increased subjects’ risk for HIV infection. How they didn’t see this until the large scale clinical trial is amazing, but I am guessing that they were as surprised by this as everyone else was.

 

But should they have been so surprised? We know that HIV infects activated CD4 cells. Some in the field have wondered whether providing a source of activated T cells by traditional vaccination would actually increase the risk of infection. So maybe that’s the answer to the question. Add to this that it has been shown that HIV preferentially infects HIV-specific CD4 cells and you can see a pattern where an immediate immune reaction might give HIV a greater chance of establishing infection. It’s hard to prove that this is what happened, but I certainly think it is a possibility.

 

The other possibility of that the vector had some synergistic effect with HIV. The vaccine was a modified adenovirus, designed to express parts of HIV. There are several examples of two viruses working synergistically. There’s even some evidence that mutants from the same virus can work together to increase pathogenesis (Vignuzzi Nature 2006—really interesting article). Maybe a viral vector isn’t the proper vehicle for the vaccine. I suppose we can’t say until Merck sorts out what happened. Either way, in spite of the vaccine failure, we are going to learn something about immunology and HIV. So I guess the trial wasn’t a total loss.

 

M. Linde