The study sample comprises 5,592 HCWs with and without regular patient contact (Fig. ). In total, 1,720 HCWs were vaccinated against pH1N1 (30.8%), including 52 pregnant HCWs (Table ). 50.4% of the study population received seasonal TIV for the season 2009/2010. Nurses had the highest vaccination rate (62.5%) for seasonal TIV but only the second highest rate (30.3% compared to 43.9% in physicians) for pH1N1 vaccination (Table ).
Fig. 1 Flow chart of the study population. Pearson’s Chi-square test for pH1N1 infection yes or no depending on pH1N1 vaccination in the group with seasonal TIV (p < 0.0001) and without seasonal TIV (p = 0.004) (more ...)
Description of the study population (n = 5,592)
Seasonal TIV and 2009 pH1N1 vaccination rates by profession
After pH1N1 vaccination, one woman experienced an anaphylactic reaction with dizziness and hypotension lasting a few minutes. No further complications were observed during the first hour after vaccination and no side effects warranting medical attention were reported. After pH1N1 vaccination, myalgia (6.9%), mild local reaction (38.0%) and strong local reaction (1.9%) were reported to the vaccination desk (Table ). No complications occurred in the 52 pregnant participants. Assessed retrospectively, 83.4% reported no side effects from the seasonal TIV, 12.3% mild local reactions and 2.9% myalgia. Strong local reactions (0.7%), fatigue (0.3%), fever (0.3%), headaches (0.1%) and lymph node swelling (0.1%) were seldom. Therefore, more side effects were reported after pH1N1 vaccination than after the 2009/2010 seasonal TIV.
Side effects of seasonal vaccination 09/10 in those 1,144 HCWs who obtained pH1N1 vaccination and side effects of pH1N1 vaccination
Between 26 October 2009 and 2 March 2010, 245 HCWs with ILS (4.4%) were referred to the pH1N1 task force in the Emergency Department (Table ). Of these, pH1N1 virus infection was confirmed in 97 cases (39.6%). Indeterminate results were observed in two out of 245 examinations (0.8%). After performing a second test, one case remained indeterminate.
The peak in ILS and pH1N1 infection in HCWs came in the 49th week of 2009. ILS occurred less often in pH1N1-vaccinated HCWs (OR 0.7; 95% CI 0.51–0.95), while the seasonal TIV showed no protective effect against ILS (OR 1.0; 95% CI 0.79–1.36). Gender was not associated with ILS (Table ). Younger workers were more likely to present with ILS (OR for ≤30 years: 2.7; 95% CI 1.69–4.42). After adjusting for vaccination, nurses (OR 2.5; 95% CI 1.53–4.09) and physicians (OR 2.0; 95% CI 1.21–3.41) had a higher risk of developing ILS than administrators.
Logistic regression for putative risk factors of influenza-like symptoms (ILS)
Out of the 97 pH1N1 infections, 91 (94%) occurred in non-vaccinated HCWs and two (2%) in HCWs vaccinated less than a week before the onset of symptoms. Overall, pH1N1 incidence was 1.7% of all HCWs, affecting 0.3% of those vaccinated and 2.4% of those not vaccinated (Table ). The seasonal TIV did not protect against pH1N1 infection (OR 1.5; 95% CI 0.98–2.27) and neither did consecutive seasonal TIV in 2008 and 2009 (Table ) (data not shown). Young HCWs were more often affected (OR for ≤30 years: 6.6; 95% CI 2.57–16.8, Table ). Nurses had an increased risk of pH1N1 infection (OR 2.7; 95% CI 1.11–6.37), while physicians had an increased but not statistically significant risk (OR 1.8; 95% CI 0.71–4.62). A total of 41 pH1N1 infections would have been expected in the vaccinated HCWs if they had not been vaccinated. The number of prevented cases is therefore 35 (41 expected minus six observed cases). The number to vaccinate in order to prevent one case of pH1N1 influenza was 49 in this particular cohort. Vaccine effectiveness (VE) was 90.4% (95% CI 73.5–97.3%).
Logistic regression for putative risk factors for pH1N1 infection
Sixty-two (64%) of the pH1N1 infected HCWs had had known contact with a pH1N1 infected individual and another 17 HCWs (17.5%) had had contact with symptomatic individuals. Fifty out of 79 potential sources of infection (63%) were patients in the hospital. The most frequent symptoms associated with pH1N1 infection were muscle or joint pain (85%), coughing (78%), fever (77%), headache (61%) and sore throat (40%). The disease was benign in its evolution in all cases.