Background: Fear of adverse reactions is one of the reasons why influenza immunization programs for hospital workers have not met wide acceptance. We conducted a study in order to compare the frequency of adverse reactions following administration of standard split-virion (VVF) and subunit-virion (VAS) influenza vaccines, mostly among hospital personnel.
Methods: Trial with volunteers who were systematically assigned to receive one of the vaccines, alternating every ten participants, during the influenza inmunization campaing 1994-95. The adverse effects were recorded by telephone interview 10 days after vaccination.
Results: Out the 182 subjects recruited, 163 were included in the study, of whom 100 received VAS and 63, VVF. Almost 13% of the participants reported any adverse effects with no significant difference between both groups (VAS: 11% and VVF: 15.6%, p = 0.38). There were also no significant differences relating to systemic and local reactions, separately. The subjects who had adverse reactions to previous influenza vaccination showed more frecuent systemic reactions (25% vs. 7.5%, p = 0.04) in a significant way.
Conclusions: Both vaccines have been proved to be very safe, only causing adverse reactions in a small proportion and very mild in every case. The VAS presents a frequency of adverse effects similar to that of VVF, in spite of containing only surface antigens. It's likely that a certain hypersensitivity to influenza vaccine exists in some people, specially to the split-virion one.