A double-blind, placebo-controlled trial published in Clinical Infectious Diseases (2012;54, June 15) examined the effects of the seasonal influenza vaccine using a true inactive placebo (saline). This is notable because many vaccine trials use “active placebos” that contain vaccine components, which can complicate interpretation of side effects.
The study followed children ages 6–15 for an average of 272 days. Sixty-nine children received the trivalent inactivated influenza vaccine (TIV; Sanofi Pasteur’s Vaxigrip), and 46 received saline placebo.
The authors reported no statistically significant difference between the vaccinated and placebo groups in laboratory-confirmed seasonal influenza infection. However, based on serologic evidence, children who received the influenza vaccine showed a lower risk of influenza infection, suggesting some protective immune response.
Unexpectedly, the study found that children who received the influenza vaccine experienced a higher rate of non-influenza respiratory infections compared with the placebo group. The incidence of these other respiratory infections was reported to be approximately 5.5 times higher in the vaccinated group than in the placebo group.
The authors proposed that this finding might be related to temporary alterations in immune responses following vaccination, sometimes referred to as “virus interference,” though the exact mechanism remains unclear. They emphasized that the study was small and that further research would be required to confirm or refute this observation.
This study does not establish causation, but it raises questions about how influenza vaccination may influence susceptibility to other respiratory viruses and highlights the importance of continued investigation into broader immune effects of vaccination.
The table below summarizes laboratory-confirmed infections observed during the follow-up period:
Vaccinated
Any seasonal influenza: 58
H1N1 influenza: 58
Total influenza cases: 116
Placebo
Any seasonal influenza: 88
H1N1 influenza: 0
Total influenza cases: 88
Non-influenza respiratory viruses:
Vaccinated
Rhinovirus: 230
Coxsackie/Echovirus: 160
Other respiratory viruses: 97
Total non-influenza viruses: 487
Placebo
Rhinovirus: 59
Coxsackie/Echovirus: 0
Other respiratory viruses: 29
Total non-influenza viruses: 88
When interpreting these data, it is important to note that case counts reflect detected viral episodes, not the number of individuals infected, and that the vaccinated and placebo groups were unequal in size (69 vaccinated vs. 46 placebo). These factors limit direct risk comparisons.
The authors reported no statistically significant difference in laboratory-confirmed influenza infection between the vaccinated and placebo groups. Serologic testing, however, suggested that vaccinated children had evidence of increased influenza-specific immunity.
The most striking finding was the higher number of non-influenza respiratory viral infections observed in the vaccinated group. When analyzed by the study authors, recipients of the trivalent inactivated influenza vaccine experienced approximately a 5.5-fold higher risk of non-influenza respiratory infections compared with the placebo group.
The authors cautioned against over-interpretation and offered several possible explanations. As they stated:
“Receipt of TIV could increase influenza immunity at the expense of reduced immunity to non-influenza respiratory viruses, by some unknown biological mechanism. Alternatively, our results could be explained by temporary nonspecific immunity after influenza virus infection, through the cell-mediated response or, more likely, the innate immune response to infection.”
In other words, the findings raise the possibility of viral interference or short-term immune trade-offs, but do not establish causation. The authors emphasized that the study was small, involved children only, and required confirmation in larger and more diverse populations.
This study highlights the complexity of immune responses to vaccination and underscores the importance of evaluating not only disease-specific protection, but also broader effects on immune function.
