Fauci and Johnston stressed that it's impossible for the vaccine itself to directly infect a person with HIV, because the adenovirus was the only pathogen included in the shot.
However, there is the possibility that vaccination might have spurred changes in the immune systems of individuals whose immune systems were already primed to fight the adenovirus. Theoretically, those immune-system changes could have made HIV infection more likely in these people if they were exposed to the virus.
"HIV replicates much better in [immune] cells that are activated," Fauci explained. For certain trial participants with a high preexisting immunity to the adenovirus, vaccination could have put their immune system on a kind of "high alert" -- activating exactly the type of CD4+ T-cells that HIV is attracted to, he said.
"Those CD4+ T-cells are then going to be very vulnerable targets for HIV when you become exposed to HIV," Fauci theorized.
But he also stressed that this only remains a theory. Research is continuing to see if the vaccine did, in fact, leave participants more vulnerable to contracting HIV.
"What we are trying to do now is to mine the data to see if we can find out any mechanistic or other circumstantial information that could help us decipher that out, and determine whether this is 'really real,'" Fauci said. That research could take up to a year to yield results, he noted.
In the meantime, Fauci and Johnston agreed that vaccine research using viral vectors should continue to go forward, albeit with an added note of caution.
If the trend seen in the study is confirmed, it could mean changes in the way the organizers of vaccine trials recruit participants in the future, they said.
"If it turns out to be biologically significant, then we will have to be very careful when using a viral vector to which people have underlying immunity, b
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