That study is
not an observation. It is a mathematical modeling of RCTs
showing benefit that the authors believe (based, again, on modeling and NOT evidence) were "implausible." My takeaway from the abstract is that a number of RCTs do in fact show benefit. Moreover, as the authors of the study you cite clearly state:
"The HCW-attributable risk and vaccine-preventable fraction both remain unknown and the NNV to achieve patient benefit still requires better understanding."
They then go on to make an astonishing philosophical claim:
"Although current scientific data are inadequate to support the ethical implementation of enforced HCW influenza vaccination..."
This is an ethical assertion, not an empirical one, and not even accompanied by a substantive ethical argument (to the extent that such a thing even exists). It should be laughed out of court.
Influenza vaccination is very safe, and although its efficacy is highly variable, I think it is trivial to formulate and counter with an ethical argument for HCW vaccination (as indeed many have done), which would, by its very nature as an ethical argument, be just that:
arguable, and highly dependent on the value-system of the individual.
Finally, I think it would be remiss of us not to link to the the excellent published response (in the same journal) equal time:
Influenza Vaccination of Healthcare Workers Is an Important Approach for Reducing Transmission of Influenza from Staff to Vulnerable Patients