Misconceptions about influenza vaccine, be it about its safety or its effectiveness, have been identified in all studies included in a recent review of attitudes and predictors of influenza vaccination among HCWs, highlighting the importance of education efforts 
Initial symptoms of RV infections are often unspecific such as fever, malaise, or myalgia. As influenza vaccine is offered when other RV are circulating (e.g., RSV), vaccinated HCWs developing symptoms within 48 h of vaccination misinterpret those signs as vaccine adverse events. In addition, the occurrence of respiratory symptoms in the months following vaccination is mistaken as vaccine failure. Other respiratory infections as the cause of such symptoms are hardly ever considered.
To diminish the arguments of fear of adverse events or perception of vaccine inefficacy, this prospective study was conducted to demonstrate to a subset of HCWs from our hospital, that severe adverse events following influenza vaccination are rare and the episodes of respiratory symptoms occurring in the first four months after vaccination are generally caused by other respiratory viruses and not by influenza virus.
As expected, no severe adverse event was observed in the present study, and the events more frequently reported, such as headache, myalgia and malaise could be related to influenza vaccine itself as well as to other causes, given their unspecificity.
In adults, the adverse event more frequently reported after intramuscular administration of inactivate vaccines is local pain, affecting 10% to 64% of the vaccinated 
. In the present study, 17.5% of the participants reported local pain.
Systemic reactions like fever, malaise and myalgia can also occur after inactivate vaccines. In the present study, the frequency of systemic AEs (over 70%) was higher than reported in previous studies. Recent publications have shown rates of systemic adverse events ranging from 30% to 59% in HWCs 
. Ideally, the subjects should have been surveyed within the first four days of vaccination. As we preferred to apply the questionnaire personally, rather than by mail or phone calls, only 49.7% of the participants were interrogated up to the fourth day, due to the great number of interviews. Thus, the high rate of systemic adverse events observed in the present series may be either an overestimation by the subjects or a consequence of the survey method applied. A recent study evaluating vaccine coverage in Korea has demonstrated that interview surveys provide more reliable information than telephone surveys, showing lower missing rates and 100% of agreement with the immunization registry record 
Anaphylaxis and neurological reactions are rare 
. The frequency of adverse events observed in the present study may be overestimated taking into account the subjectiveness of self-reported unspecific symptoms.
As HCWs are aware of vaccine adverse events and fear its consequences, it is comprehensible that these events will be more frequently reported by them than by general population. Another study conducted in the same hospital demonstrated that HCWs reported significantly more adverse events (52.9%) than the elderly (25.3%) 
. The higher frequency of adverse events reported by HCWs surveyed in the first five days of vaccination, as compared with those surveyed after the fifth day, may suggest that people may be more predisposed to remember any symptom possibly associated with the vaccine if inquired within the first days of vaccination. On the other hand, we believe that if the adverse events were severe or important, they would not be missed if inquired after 6 to 10 days.
Interestingly, we observed that some sectors showed significantly higher rates of AE than others, supporting the subjectivity of the information. Also, this data may suggest a mouth to mouth effect among sector coworkers influencing the self-report of AE. Among HCWs, the belief that coworkers take influenza vaccine influences the vaccine uptake. Thus, it is possible that the same occurs concerning to adverse events.
Continued education of health professionals is essential to highlight not only the epidemiological importance of the vaccine, but also its safety and the low risk of severe adverse events.
Our study also demonstrated that the respiratory symptoms occurring in the months following influenza vaccination were more frequently caused by other respiratory viruses and generally do not mean vaccine failures.
One limitation of our study is that in only 93 of the 192 episodes of respiratory symptoms (48.4%) the subjects agreed with NW sampling. NW sampling is a simple but uncomfortable procedure and this fact may explain why some HCWs preferred not to get tested during working hours. One could argue that influenza cases could be missed among those not tested. However, we believe that this loss has not affected our results as the frequency of ILI was similar between those who agreed with sampling and those who did not (, p
The incidence density of other respiratory viruses was 2.4 times greater than incidence density of influenza. Probably, this difference would be even greater if real time PCR was also performed to increase the sensitivity of the diagnosis of other respiratory viruses as well. In addition, more cases of other RV infections would be diagnosed if a larger number of professionals were tested, increasing the difference between the incidence density of influenza and other RV.
Influenza infection is characterized by the abrupt occurrence of fever, headache, myalgia, and dry cough. During influenza season, the presence of these symptoms is highly predictive of influenza infection and summarizes the case definition of influenza-like illness
(ILI), which has been used worldwide for influenza surveillance purposes. However, the sensitivity and positive predictive value of such definition can vary greatly depending on the co-circulation of other respiratory viruses in the community 
. Indeed, Bellei et al. have recently reported that 70% of ILI cases in the city of São Paulo were caused by other agents, mainly rhinovirus, which peaks along with influenza 
. Similar results have been previously published by other authors 
In our series, influenza cases in vaccinated HCWs were mild and occurred significantly earlier following vaccination in comparison to other respiratory viruses. This finding may be explained by the marked seasonality of influenza in São Paulo city as reported previously 
, peaking in early winter and coinciding with the initial period of the study.
The effectiveness of influenza vaccines is related predominantly to the age and immune competence of the vaccinee and the degree of similarity between the viruses in the vaccine and those in circulation. Vaccine effectiveness in preventing laboratory-confirmed influenza illness when the vaccine strains are well matched to circulating strains is 70–90% in randomized, placebo-controlled trials conducted among children and young healthy adults, but is lower among elderly or immunocompromised persons 
. In adults ≥65 years old, the efficacy of influenza inactivate vaccine varies from 30% to 40% 
Trials that measure laboratory-confirmed influenza virus infections as the outcome are the most persuasive evidence of vaccine efficacy 
. In the present study, only five of the 337 vaccinated HCWs (1.5%) acquired influenza.
In accordance with the educational nature of our study, we considered all cases of influenza as vaccine failures, since vaccinated health personnel look forward to be protected against influenza. Molecular characterization of influenza cases was not performed to check for possible mismatches between circulating viruses and vaccine strains, which could possibly justify those failures.
Our study demonstrated that the fear of severe adverse events seems unjustified as well as the perception of vaccine inefficacy. URI following influenza vaccination were generally caused by other respiratory viruses and not by influenza.
In times of pandemic influenza A H1N1 and widespread vaccination, healthcare and emergency medical services personnel are among the priority groups recommended to receive the H1N1 influenza vaccine. It is time to overcome definitively the misconceptions about the vaccine as well as the fear of adverse events. So far, the vast majority (93%) of adverse events reported to VAERS after receiving the trivalent 2010–2011 influenza vaccine, were classified as “non serious”, e.g., soreness at the vaccine injection site 
We believe that the educational nature of the present study may persuade HCWs to develop a more positive attitude to influenza vaccination.