This cross-sectional survey took place from November 17 to 25, 2009, shortly after the mass vaccination campaign had started in the general population (November 12), i.e. twelve weeks after the influenza-pandemic occurred in France and a week before the peak (November 23–29 – Week 48) as surveillance epidemiological data revealed subsequently 
. Overall, the acceptability of A/H1N1 vaccination was low at 17.0% (CI 95%, 15.5% to 18.7%) among 2,167 respondents representative of the French adult population aged 18 to 64. The majority of the French general population did not associate A/H1N1 influenza-pandemic illness with a serious threat. Only 35.5% of respondents perceived A/H1N1 influenza illness as a severe disease and 12.7% had experienced A/H1N1 cases in their close relationships with higher acceptability of vaccination (p<.0001 and p
.006, respectively). In comparison to 26.0% respondents who did not consult their primary care physician in the prior six months, acceptability was significantly higher among 8.0% respondents who were formally advised to get vaccinated by their primary care physician, and lower among 63.7% respondents who were not advised to get vaccinated (respectively: 15.8%, 59.5% and 11.7%- p<.0001). Among 1,798 respondents who refused vaccination, 71.2% expressed concerns about vaccine safety.
We found that risk perceptions of the A/H1N1 influenza-pandemic were strongly correlated to the acceptability of vaccination in the general population. It confirms findings from previous surveys conducted worldwide about attitudes and behaviors toward vaccination against seasonal flu 
, the highly pathogenic A/H5N1 influenza virus 
, and more recently the A/H1N1 influenza-pandemic 
. However, the majority of the French general population did not associate A/H1N1 influenza-pandemic with a serious threat, albeit a week before the pandemic peak 
The substantial impact of other determinants illustrates that while the perceived severity of the A/H1N1 influenza-pandemic may be a sufficient condition for getting vaccinated in a mass vaccination campaign, it is not a necessary one. We found that individual characteristics including male gender, older age, and previous receipt of seasonal influenza vaccine were independent predictors of the acceptability of A/H1N1 pandemic vaccination. The same individual characteristics were similarly associated with the acceptability of A/H1N1 pandemic vaccination in other countries 
, but also with seasonal vaccine uptake in the whole French at-risk population aged less than 65 
. These findings suggest that prior beliefs and attitudes toward seasonal influenza vaccination are major leverages to increase the uptakes of influenza-pandemic vaccination in the general population.
However, we found that acceptability of A/H1N1 pandemic vaccination was as low as 17.0% among the French adult population, and concerns about A/H1N1 pandemic vaccine safety were the main reason quoted by 71.2% respondents who denied being vaccinated. In a Canadian qualitative study among health care professionals and the general public, the authors found that individuals were hesitant to accept pandemic vaccines and that “concerns about using new vaccines during a pandemic differ from concerns about using established products in non-crisis situations” 
. For an emerging public health threat that diffuses very quickly, as it has been the case for the A/H1N1 influenza-pandemic, perceptions of the benefits and risks of vaccination may continuously evolve. Our results primarily suggest that the general population was not reassured that A/H1N1 pandemic vaccines were safe. It calls into question the information received by the general population at time of the survey, and what factors may have worsened the perception that A/H1N1 pandemic vaccines are unsafe.
At time of the survey, the A/H1N1 influenza-pandemic had attracted massive media coverage in France, albeit in two opposite directions. On the one hand, the severity of A/H1N1 2009 influenza illness was stressed by daily reports of fatalities in the news media (i.e. 357 hospitalizations in intensive care units (ICUs) and 68 deaths at time of the survey) 
, frequent messages from public health authorities, and personal appearances in the media of the Ministry of Health and the Head of State in order to motivate people's compliance with the mass vaccination campaign. On the other hand, the safety of A/H1N1 pandemic vaccines was scrutinized by the media with regard to the risk of Guillain-Barré syndrome, the limited knowledge about adjuvanted vaccines accounting for almost all doses available in France, the accelerated authorization procedure to market pandemic vaccines and the actual motivations of pharmaceutical firms, while the unclear number of vaccine injections called their protective efficacy into question.
Although the public's perception of a health risk usually increases with its coverage in the news media 
, this general trend may indeed be counteracted if this media information is dissonant 
, and if daily personal experience does not confirm the threat 
. Previous population surveys in the US
and the UK
have emphasized that after an initially high level of risk perception, levels of anxiety waned along with the perception of the A/H1N1 influenza-pandemic as an immediate threat and that tackling the perception that the outbreak has been “over-hyped” may be difficult. In our study, 12.7% respondents reported a case of A/H1N1 flu in their close environment. The fact that these respondents had a significantly higher acceptability of A/H1N1 pandemic vaccination suggests that they saw the A/H1N1 influenza-pandemic as a real threat in concordance with messages from public health authorities. On the contrary, dissonance may have grown in the vast majority of the general population who had no (72.3%) or indirect (15.0%) experience with A/H1N1 influenza-pandemic.
As a consequence, 74% respondents looked for medical advice about A/H1N1 pandemic vaccination, an estimate above the expected number of consultations for a similar period 
. Previous studies have shown that behaviors, attitudes, and advice from primary care physicians were strongly associated with their patients' immunization behavior for seasonal influenza 
as well as the “swine flu” in the 1976 mass vaccination campaign in the U.S. 
. We found consistently that a positive advice from a primary care physician was a major determinant of the acceptability of A/H1N1 pandemic vaccination. However, 63.7% respondents were not advised to get vaccinated with significantly lower acceptability of A/H1N1 pandemic vaccination. First and foremost, this finding is in accordance with the low uptake rate (10.9%) reported at time of the survey for health care professionals who were the first priority group to access pandemic vaccines 
. To the extent that 62% of general practitioners were willing to get vaccinated during the summer of 2009 
, future studies should explore whether their behaviors and attitudes toward A/H1N1 pandemic vaccines did evolve negatively as a result of risk communication of public health authorities and/or their dismissal from the mass vaccination campaign decided on August 21, 2009 
. Assumingly, the decision to strictly administer A/H1N1 pandemic vaccines in ad hoc vaccination centers had further increased dissonance in the general population since it was in sharp contrast with the past experience of the general population learnt from seasonal flu vaccination that is mainly prescribed and administered by primary care physicians in France 
, as well as policies adopted by neighboring countries, like Belgium, Germany, and the UK 
Finally, we found that parents had a higher acceptability of A/H1N1 pandemic vaccination for themselves than other adults without children. Further analysis showed that such higher acceptability was mediated by the perception of A/H1N1 influenza-pandemic illness as a “severe” or “very severe” disease with an increased acceptability depending on the number of children in the household. However, parents who denied vaccination for themselves expressed significantly more concerns about vaccine safety than other adults without children (76.5% vs. 66.6%, respectively; p<.0001). Quite logically, parents were reluctant to get their children vaccinated; only a quarter of parents accepted vaccination for their children but not for themselves. Future studies should address more specifically knowledge, attitudes and behaviors of parents about pandemic vaccination of their children since children are the most important drivers of influenza infection and may be targeted for pandemic vaccination before their parents 
Our study is subject to a number of weaknesses. The advantage of our Web-based sampling strategy is the ability to quickly deploy a survey and thereby track responses in near real-time knowing that risk perceptions and attitudes toward pandemic vaccination may continuously evolve 
. The possible disadvantage of this strategy is a sacrifice of population representativeness. A non-coverage bias is limited by the quite high Internet coverage in the French adult population (estimated at 67% in 2008), while coverage rates are the highest in our target population of adults aged 18 to 64 
. The representativeness of online data collection is also established to the extent that it follows the procedure used in this survey, i.e. stratified random sampling in a pre-existing large representative panel of the whole population 
Although we cannot unequivocally rule out the existence of selection bias in our online sample, our analyses are consistent with the view that our sample is representative of the French adult population aged 18 to 64 as compared to previous surveys conducted in random samples with use of traditional methods for data collection (face to face or phone interviews): 1) random sampling in our survey was stratified to match French official census statistics for gender, age, occupation, household size, size of the population in the area of residence, and region 
; 2) 22% of respondents received seasonal influenza vaccination at least once in the prior three years consistent with national uptake rates around 22–24% over recent years 
; and 3) usual explanatory factors for seasonal flu uptake were also consistently found to associate with acceptability of A/H1N1 vaccination (i.e. male gender, older age, previous vaccination against seasonal flu, groups at risk for influenza complications) 
Although such online survey shares with other survey methods the general limitations of results based on respondent's self-declarations, it is well established that self-administered questionnaires tend to yield fewer reports in the socially desirable direction than do interviewer-administered questionnaires, and a recent study suggested that online surveys may have the lowest social desirability bias 
. In particular, the validity of our results was further supported by actual immunization behaviors reported in official statistics: the low uptake of A/H1N1 vaccine among health care professionals (10.7%; CI 95%: 5.8% to 17.7%), who were the first priority group to access vaccines on October 20, 2009, was similar to the actual uptake rate (10.9%) reported at time of the survey 
; the low acceptability of A/H1N1 vaccination in the French general population (17.0%; CI 95%, 15.5% to 18.7%) predicted the low coverage rate (2.7 million people, i.e. 7.1% coverage in the population aged 18 to 60) reported on February 28, 2010 
The uptake of A/H1N1 pandemic vaccines appears to be very low in France as compared to some other European Union and North American countries that have undertaken a mass vaccination campaign 
. While risk perceptions of A/H1N1 influenza-pandemic were expectedly found to drive immunization behaviors, the majority of the adult population expressed concerns about pandemic vaccines' safety and refused vaccination for themselves and their children. As evidence by this study and others, risk communication of public health authorities should primarily focus on reassuring the general population that pandemic vaccines are safe 
. In addition, our study shows that the implementation of a mass vaccination campaign and the particular role given to primary care physicians were major factors to achieve a successful pandemic vaccination campaign. On January 11, 2010, the French Ministry of Health reversed its policy and authorized primary care physicians to administer A/H1N1 vaccines. While such policy change should contribute to increase significantly uptake rates among priority groups at risk for A/H1N1 influenza-pandemic complications, it may have occurred too late to change uptake rates in the general population at a time the fist wave of the influenza-pandemic ended