Self-reported pH1N1 influenza vaccination coverage for Ontario

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by Julie Foisy, Laura C. Rosella, Ruth Sanderson, Jemila Seid Hamid, Badal Dhar and Natasha S. Crowcroft

The mass vaccination campaign against pH1N1 that Canada undertook in the fall of 2009 was the largest ever conducted in the country.  The vaccine became available to Ontario residents October 26, and by December 6, the province had distributed enough doses to cover 81% of the population.1  Because the vaccine was delivered through public health units, several methods were used to document coverage, making an overall Ontario estimate challenging to compute.  This report, based on an existing random digit-dialling telephone survey, provides an overview of self-reported pH1N1 vaccination uptake for Ontario (see The data).

One in three

Based on the results of the Rapid Risk Factor Surveillance System (RRFSS) survey, pH1N1 vaccination coverage for Ontario from the end of October 2009 to the end of April 2010 was 34.5% overall:  33.3% for adults aged 18 or older, and 38.6% for children and adolescents younger than age 18 (Table 1).  Sensitivity analyses that excluded respondents missing age information yielded almost the same percentages:  34.4% overall, 33.0% for adults, and 38.5% for children and adolescents.

Table 1 Percentage vaccinated against pH1N1, by selected characteristics, household population, Ontario, October 2009 through April 2010Table 1 Percentage vaccinated against pH1N1, by selected characteristics, household population, Ontario, October 2009 through April 2010

These results are broadly in line with estimates in two earlier reports.  The Chief Medical Officer of Health's report, released in June 2010, used data from a weekly Ipsos Reid poll conducted from October 2009 to mid-January 2010 to determine pH1N1 vaccination coverage.  According to this poll, approximately 39% of Ontarians had been vaccinated,2 somewhat above the estimate in the current analysis. 

Based on data from the RRFSS survey, 33% of Ontario residents aged 12 or older received the pH1N1 vaccine.  For the same age group and the same interview period (January through April 2010), the Canadian Community Health Survey reported 32.2%.3

The vaccination status of adults living in households with at least one child (35.2%) did not differ significantly from that of adults in households with no children (33.8%).

Youngest and oldest

The percentage of individuals vaccinated varied by age, with the highest levels at the extreme ends of the age range (Figure 1).  Fully 57% of seniors aged 65 or older were vaccinated, as were more than 40% of adults aged 55 to 64 and children aged 0 to 11.  By contrast, around 20% of adults aged 18 to 29 reported having been vaccinated. 

Figure 1 Percentage vaccinated against pH1N1, by age group, household population, Ontario, October 2009 through April 2010Figure 1 Percentage vaccinated against pH1N1, by age group, household population, Ontario, October 2009 through April 2010

Higher percentage of women

Overall, a significantly higher percentage of women than men aged 18 or older were vaccinated (35.7% versus 30.9%; p<0.0001), a pattern that prevailed in most age groups (Table 2).  At ages 40 to 44, the difference (39.0% versus 26.0%) was statistically significant.

Table 2 Percentage vaccinated against pH1N1, by sex, age group and chronic condition, household population aged 18 or older, Ontario, October 2009 through April 2010Table 2 Percentage vaccinated against pH1N1, by sex, age group and chronic condition, household population aged 18 or older, Ontario, October 2009 through April 2010

Chronic conditions

One in five (20.3%) adults aged 18 or older reported a chronic condition that, according to the  National Advisory Committee on Immunizations,4 put them at high risk of complications from pH1N1 infection.  These people were significantly more likely than those without a high-risk chronic condition to have been vaccinated:  45.4% versus 30.2%.  While women were generally more likely than men to have been vaccinated, for those with high-risk chronic conditions, the percentages were almost the same (Table 2).  

November peak

The pH1N1 vaccine was available to Ontario residents from October 26, with nearly 23% of people who were vaccinated receiving their shot in the first week of the campaign.  Almost 44% of vaccinated Ontarians received their vaccination in November (Figure 2).

Figure 2 Percentage vaccinated against pH1N1, by month, household population, Ontario, October 2009 through April 2010Figure 2 Percentage vaccinated against pH1N1, by month, household population, Ontario, October 2009 through April 2010

In December, a further 17.3% of vaccinated people received the pH1N1 vaccine.  A number of factors may have contributed to the decline observed in December.  For example, by the time priority was extended to include the general population (in December), many may have felt that obtaining the vaccination was not necessary.  Results from a study in Australia showed that a high percentage of people were not vaccinated for this reason.5  In the January to April 2010 period, 5.4% of those who received the pH1N1 vaccine had their shot.  

Conclusion

Estimates of vaccination coverage are important in the evaluation of Ontario's pH1N1 prevention program.  Identifying the extent of coverage offers important information on the potential burden of future waves of pH1N1.    Individuals at highest risk of complications from pH1N16—young children, the elderly and those with chronic conditions—had high rates of vaccination.  This suggests that a large number of Ontario residents who were at highest risk were protected as a result of having been vaccinated.

Acknowledgements

The authors acknowledge the Rapid Risk Factor Surveillance System, all the health units that participated, and the Institute for Social Research at York University.