| A public health officer of the Direction Départementale de
l'Action Sanitaire et Sociale des Pyrénées Atlantiques in France was notified of an
outbreak of acute respiratory illness among residents of a nursing home on 4 December,
1995. Over 50 of the 69 residents had been ill in the preceding three weeks and six had
been admitted to hospital. An epidemiological and microbiological investigation was
conducted to identify the cause of the outbreak and associated risk factors and to measure
vaccine efficacy (VE). Materials and
methods
A clinical case was defined as a resident of the nursing home
who had fever (axillary temperature) > = 38°C and respiratory symptoms (cough
and/or sputum production) between 11 November and 15 December 1995. Demographic and
clinical data (underlying conditions, symptoms and signs) and histories of immunisation
against influenza were obtained about all residents by reviewing the nursing log book and
medical charts and by interviewing the chief nurse on 19 December.
Sputum and serum specimens were collected on 6 December from
patients in hospital. Standard cultures and serological tests for viruses and bacteria
were performed in the hospitals where they had been admitted. Cultures for influenza virus
(egg) and adenovirus (standard and immunoperoxidase), and serological tests (complement
fixation) for influenza A and B, adenovirus, parainfluenzae 1 and 3, and respiratory
syncytial virus were performed at the virology laboratory, Centre Hospitalier
Universitaire, Bordeaux. Data were analysed using a retrospective cohort study design.
Vaccine efficacy (VE) was calculated for the above case definition (case definition 1) and
a more specific one (case definition 2, which required a higher body temperature > =
38.5° C but the same respiratory symptoms). VE was estimated as 1 - (attack rate [AR]
among vaccinated residents / AR among unvaccinated residents).
Results
Three residents were excluded from the analysis because they
had been admitted to hospital for unrelated conditions during the epidemic. The study
population therefore consisted of 66 people with a mean age of 80 years (range 58-101
years, median 82), 25 of whom were men. Forty-four had one or more of the following
chronic conditions: neurologic or psychiatric diseases 24; diabetes 8; chronic bronchitis
6; cardiovascular diseases 5; neoplasms 3; arthritis 2. Fifty-two residents had received
one of two brands of a polyvalent influenza vaccine on 10 October.
Forty-three residents became ill (AR: 65%). Their mean
temperature was 38.8° (38.0° to 40.3°C). Six cases were admitted to hospital for
respiratory complications an average of seven days after becoming ill (range 0 to 14
days). One patient, who had not been admitted to hospital, died seven days after becoming
ill. Acute and convalescent serum specimens were collected from five patients: two
seroconverted (< 40 to 640), and one fourfold rise of complement fixing antibody titres
to influenza A antigen was observed. One of the patients in whom seroconversion was
observed had not been vaccinated. Cultures were not performed for any patient at the onset
of the illness. All other laboratory investigations were negative.
The epidemic curve (figure) spans five weeks, but 39/43 cases
arose within 11 days. The attack rate was uninfluenced by sex (males, 15/25, 60%; females,
28/41, 68%), age (< = 80, 15/24, 63%; > 80, 28/42, 67%) and location in the
home (ground floor, 18/25, 72%; first floor, 21/34, 62%; and the other building, 4/7, 57%)
The vaccine efficacy (VE) was 30% (95% confidence interval
[CI] 5-49%) for case definition 1, compared with 43,5% (95% CI 10-65) for the more
specific case definition 2 (table). VE was the same in residents of both floors and was
unaffected by which of the two vaccines they had received. VE was higher in people aged 80
years and under than in older residents (45% [95% CI 20-63] and 19% [95% CI 27-48] for
case definition 1 and 55% [95% CI 8-78] and 35% [95% CI 20-65] for case definition 2,
respectively).
Table : Vaccine efficacy by case definition
and vaccine used, nursing home influenza outbreak, Pyrénées Atlantiques, Nov.-Dec. 1995
| Vaccination status |
Total |
Cases |
Attack rate% |
VE% |
95% CI |
| |
| Case definition 1 |
|
|
|
|
|
|
| Vaccine 1 |
31 |
17 |
55 |
36 |
6-56 |
| Vaccine 2 |
21 |
14 |
67 |
22 |
-13-46 |
| Any vaccine |
52 |
31 |
60 |
30 |
5-49 |
| No vaccine |
14 |
12 |
86 |
ref. |
- |
Case definition 2
(fever > = 38.5° C) |
|
|
|
|
|
|
| Vaccine 1 |
31 |
13 |
42 |
41 |
0-65 |
| Vaccine 2 |
21 |
8 |
38 |
47 |
0-72 |
| Any vaccine |
52 |
21 |
40 |
43 |
10-65 |
| No vaccine |
14 |
10 |
71 |
ref. |
- |
Discussion
Influenza surveillance data from the French physician
sentinel network show that an outbreak of influenza occurred in the Bordeaux area from
early November 1995 to mid-January 1996. The most prevalent strain was influenza virus A
(H3N2).
As no viral culture was performed at the early phase of the
outbreak, the diagnosis in this outbreak was confirmed serologically. Antibodies to
influenza virus can be detected 10 to 15 days after vaccination. An increase in antibody
titres observed six to eight weeks after vaccination should be attributable to infection.
One seroconversion was seen, however, in a resident who had not been vaccinated. As only
three cases were confirmed, we used a clinical case definition to assess VE. A clinical
case definition based on fever may lead to an overestimate of VE, because vaccinated
cases, if they become ill, might have milder influenza symptoms, including fever (1).
The attack rate in this outbreak was high despite a vaccine
coverage of about 80%. VE was between 30% and 43%, however, which is comparable to other
estimates (1,2). The higher VE obtained with case definition 2 tallies with the decreased
risk of misclassification of cases when a more specific case definition is used. The small
population did not allow us to confirm the observation that influenza vaccine reduces the
complications and mortality associated with influenza.
There are few contraindications to vaccination against
influenza and a high coverage (including health care workers and other staff members) in
closed communities at risk can help to reduce the intensity of transmission (3).
Antiviral prophylaxis (amantadine or rimantadine) if used
promptly reduces influenza morbidity (AR) by 80% (4) and has been used successfully to
control outbreaks of influenza in nursing homes. These drugs are either not available or
used for outbreak control in France and most other European countries. |