Health officials worry that anti-vaccine activists will seize upon Powell's death to make the claim that vaccines don't work.
Sounds like a very scientific worry. </sarcasm>
The worry of the officials noted above is a self-evidently unscientific worry. Those so-called medical officials are not trusting science to refute "anti-vaccine activists," which only reasonably would mean that the science would not support the officials' position, or might even show the officials to be wrong. That certainly is the apparent logic of their worry!
The portent seems to be that anti-vaxxers are closer to what the science would argue than to what these health officials claim and follow. Good! Let the officials suffer for their non-science!