American Journal of Respiratory and Critical Care Medicine

There is little evidence that influenza vaccination reduces asthma exacerbations. We determined whether influenza vaccination is more effective than placebo in 6–18-year-old children with asthma. We performed a randomized, double-blind, placebo-controlled trial. Parenteral vaccination with inactivated influenza vaccine or placebo took place approximately November 1, and children were followed until April 1 of the next year. Airway symptoms were reported in a diary. When symptom scores reached a predefined level, a pharyngeal swab was taken. Primary outcome was the number of asthma exacerbations associated with virologically proven influenza infection. Three hundred forty-nine children were assigned placebo, and 347 were assigned vaccine. Pharyngeal swabs positive for influenza were related to 42 asthma exacerbations, 24 in the vaccine group and 18 in the placebo group, a difference of 33% favoring placebo (31% after adjustment for confounders; 95% confidence interval, −34% to 161%). Influenza-related asthma exacerbations were of similar severity in both groups; they lasted 3.1 days shorter in the vaccine group (95% confidence interval, −6.2 to 0.002 days, p = 0.06). We conclude that influenza vaccination did not result in a significant reduction of the number, severity, or duration of asthma exacerbations caused by influenza. Additional studies are warranted to justify routine influenza vaccination of children with asthma.

Infections of viral origin, including influenza, cause exacerbations in patients with asthma (1, 2). Most guidelines support the opinion that influenza vaccination should be given to patients with asthma, including children (3).

Over the years, the necessity of vaccinating patients with asthma has been discussed (46). A Cochrane systematic review on influenza vaccination in people with asthma concludes that because of the lack of randomized trials, the evidence to assess the benefits and risks of influenza vaccination is insufficient (7). Inhalation medication for relief and maintenance therapy has made it easier to control asthma, and exacerbations can be prevented or suppressed; thus, patients can lead normal lives.

The proportion of children with asthma that is vaccinated varies widely and is lower than in the older population (8). This is partly due to doubts among patients and doctors about the benefit of the vaccination and the lack of evidence for the protective effect of influenza vaccination against asthma exacerbations (9, 10).

An answer to the question of whether influenza vaccination can prevent exacerbations in children with asthma is necessary to either validate or modify current guidelines and may enhance evidence-based practice.

We conducted a study to investigate whether influenza vaccination in children with asthma prevents asthma exacerbations provoked by influenza infection. Some of the results of this study have been previously published in the form of abstracts (1113).


We performed a randomized, double-blind, placebo-controlled parallel trial in children 6–18 years of age with asthma who were recruited through family physicians in the city of Rotterdam and its surroundings during the winter seasons of 1999–2000 and 2000–2001. Patients could only participate for one season. Exclusion criteria were other chronic diseases, allergy to chicken protein, and insufficient understanding of the Dutch language (see online supplement for further details). We informed parents and children orally and in written text about the purpose and contents of the study and obtained informed consent for those willing to participate. The Medical Ethical Committee of Erasmus MC-University Medical Center Rotterdam approved the study.


In both seasons, research nurses vaccinated all participants between October 25 and November 24 with either inactivated influenza vaccine or placebo. Vaccine details are provided in an online supplement.


We aimed to answer the question of whether influenza vaccination is more effective than placebo in preventing asthma exacerbations caused by influenza infections among children with asthma who were 6–18 years of age. We tested the null hypothesis that placebo is as effective as influenza vaccination in preventing asthma exacerbations caused by influenza infections.


Primary outcome was the number of asthma exacerbations associated with virologically proven influenza infection. Secondary outcomes were among others the duration and severity of these asthma exacerbations, adverse effects of the vaccination, including airway symptoms, the number, duration, and severity of all asthma exacerbations and of influenza-related and all upper respiratory tract episodes. Additional outcomes are listed in the online supplement.

Respiratory Tract Symptoms and Episodes

Following the method described by Johnston and colleagues (2), participants were asked to fill in a diary, starting the day after vaccination, and daily score symptoms of upper and lower respiratory tract, use of medication, physician visits, and other use of medical facilities. An episode of either upper respiratory tract or lower respiratory tract symptoms was defined as 2 or more days with symptom scores above the median for that child preceded by at least 1 day at or below the median and followed by at least 2 days at or below the median (compare Johnston and colleagues) (2) (Figure 1)

. An asthma exacerbation was defined as an episode of lower respiratory tract symptoms. Further details are provided in the online supplement.

Virologic and Serologic Assessments

Throat swab material was analyzed by culture, immunofluorescence, and reverse transcription-polymerase chain reaction. A full description of the methods used is provided in the online supplement.

We took blood samples (4 cc) before vaccination (sample 1), 14–21 days afterward (sample 2), and at the end of the season, around the 1st of April (sample 3). The presence of influenza virus–specific antibodies was performed by a hemagglutination inhibition assay (details are given in the online supplement).

A fourfold increase in postseason serum antibody titer (comparing sample 2 and 3) was considered an indication for an influenza virus infection.

Sample Size

We wanted to be able to detect a 50% reduction in the number of asthma exacerbations caused by influenza with α = 0.05 and 80% power. Assuming a 30% influenza incidence in the placebo group and 50% of these infected children reacting with an exacerbation, we aimed at a total number of 600 children, that is, 300 children per season.

The online supplement provides additional information on sample size calculation, randomization procedure, blinding, assessment of adverse effects, and statistical analyses.

Altogether, 144 family practices (200 physicians) participated. They selected 3,220 children and informed these children and their parents by letter about the objectives of our study. In total, 1,365 children were willing to participate. After inclusion and informed consent, 697 children were randomized (first season, 297; second season, 400) (Figure 2)

. One participant, after having received the study intervention, was mistakenly vaccinated by her family doctor as well and excluded because of protocol violation. Of the remaining participants, 347 were vaccinated with influenza vaccine and 349 with placebo. In each group, 344 participants provided diary data for primary outcomes. The proportion of diary days for which no data were available was 4.7%. Because the number of missing diaries was very small, we decided not to impute data but to exclude these children from the analysis for these outcomes.

Baseline Characteristics

The groups were similar for baseline characteristics except for the history of allergy (Table 1)

TABLE 1. Baseline characteristics of the study population (n = 696)

Values Are Percentages Unless Otherwise Stated

 (n = 347)

 (n = 349)
Age in years, mean (SD)10.5 (3.2)10.6 (3.3)
Female, n48.143.6
Age of onset of asthma in years, mean (SD)3.5 (3.8)3.3 (3.4)
Asthma complaints more than once a week previous 12 mo15.618.6
Only maintenance medication in previous 12 mo29.124.4
Relief and maintenance medication in previous 12 mo59.964.8
FEV1% predicted, mean (SD)89.5 (16.1)88.9 (16.0)
Ever treated by asthma specialist40.746.6
Still treated by asthma specialist25.927.1
Ever hospitalized for asthma16.217.0
Vaccinated previous season for influenza51.453.0
Ever vaccinated for influenza53.656.7
If vaccinated before, number of seasons, mean (SD)2.9 (1.9)2.8 (1.8)
History of allergy73.566.1
Proven allergy for pollen, house dust mite, or pets47.344.7
History of eczema50.648.4
Seasonal influence on asthma74.477.0
Family history of asthma78.777.1
Exposure to cigarette smoke at home30.333.2
Current smoker2.62.6
Pets at home58.259.6
Child born in the Netherlands
. Almost 90% of children had used maintenance medication for asthma during the previous 12 months; 55% of the children had been vaccinated against influenza before.

Influenza Activity

In our study, in the season 1999–2000, the first influenza positive swab was taken on December 17, 1999, and the last one on January 20, 2000. All were influenza A virus, and the four cultures from these swabs all yielded H3N2. Dutch primary care sentinel stations detected influenza A H3N2 from week 45 (November 8) in 1999 until week 8 (February 21) in 2000 (14).

In the season 2000–2001 in our study, the first influenza A virus was detected on November 21, 2000, and the last on March 23, 2001. All cultures yielded H1N1. Three influenza B positive swabs were obtained March 7–13, 2001. This season sentinel stations reported influenza A H1N1 from week 50 (December 11) in 2000 until week 8 (February 19) in 2001. Influenza B was spotted in week 8 (February 19–26, 2001) (15).

Primary Outcome

We received 486 calls (from 347 children) to report a symptom score of 4 points or more, 251 from the vaccine group and 235 from the placebo group. In 44 of the throat swabs collected on these calls, influenza virus was detected (Table 2)

TABLE 2. Influenza viruses detected by either culture, immunofluorescence, or polymerase chain reaction in pharyngeal swabs collected on calls for airway symptoms



Absolute Numbers
Total number of calls112111139124486
Influenza A2 (2)6 (7)20 (21)13 (9)41 (39)
Influenza B
1 (1)
2 (2)
3 (3)

Number of asthma exacerbations linked to positive swab results are in parentheses.

, of which 38 (21 in vaccine group and 17 in placebo group) could be linked to one or more asthma exacerbations, our primary outcome measure. This resulted in 42 influenza-related exacerbations. Of these, 24 were found in the group that received influenza vaccination and 18 in the placebo group, a difference of 33% (crude) or 31% after adjustment (95% confidence interval ranging from a 34% reduction to a 161% increase) (Table 3)

TABLE 3. Episodes of asthma and upper respiratory tract symptoms


Absolute Numbers
Ratio V/P
 (95% CI)
Ratio V/P
 (95% CI)
Number of episodes
Influenza-related asthma exacerbations24181.33 (0.69 to 2.57)0.391.31 (0.66 to 2.61)0.44
Influenza-related URT episodes20181.11 (0.59 to 2.09)0.750.95 (0.49 to 1.82)0.87
All asthma exacerbations1,1561,1551.00 (0.88 to 1.13)0.991.00 (0.88 to 1.12)0.99
All URT episodes1,4191,3511.05 (0.94 to 1.17)0.391.03 (0.92 to 1.15)0.59
Percentage of children with episodesOdds ratio V/P (95% CI)Odds ratio V/P (95% CI)
Influenza-related asthma exacerbations5.8%4.9%1.19 (0.61 to 2.31)0.611.24 (0.62 to 2.48)0.54
Influenza-related URT episodes5.5%5.2%1.06 (0.55 to 2.05)0.871.16 (0.57 to 2.35)0.68
All asthma exacerbations85.5%90.1%0.65 (0.41 to 1.03)0.060.67 (0.41 to 1.08)0.10
All URT episodes92.2%92.7%0.92 (0.52 to 1.62)0.770.84 (0.46 to 1.52)0.57
Duration of episodes (days) mean, SDDifference V–P (95% CI)Difference V–P (95% CI)
Influenza-related asthma exacerbations9.2 (3.6)11.2 (5.3)−2.0 (−4.9 to 0.9)0.19−3.1 (−6.2 to 0.0)0.06
Influenza-related URT episodes8.1 (3.6)8.0 (3.7)0.1 (−2.3 to 2.4)0.95−0.6 (−2.9 to 1.5)0.52
All asthma exacerbations6.8 (5.0)7.6 (5.0)−0.7 (−1.3 to −0.1)0.03−0.8 (−1.4 to −0.2)0.01
All URT episodes6.5 (4.7)6.6 (4.0)−0.2 (−0.7 to 0.3)0.54−0.3 (−0.7 to 0.1)0.19
Severity of episodes (symptom score)Difference V–P (95% CI)Difference V–P (95% CI)
Influenza-related asthma exacerbations4.7 (2.7)6.4 (3.1)−1.7 (−3.5 to 0.1)0.08−1.3 (−3.1 to 0.4)0.14
Influenza-related URT episodes7.4 (2.7)7.1 (3.7)0.4 (−1.7 to 2.5)0.72−0.4 (−2.4 to 1.6)0.70
All asthma exacerbations3.0 (1.7)3.1 (2.0)−0.1 (−0.4 to 0.2)0.42−0.1 (−0.4 to 0.1)0.39
All URT episodes3.3 (1.7)3.4 (2.2)−0.0 (−0.3 to 0.2)0.89−0.0 (−0.4 to 0.3)0.81
Percentage of days with symptomsDifference V–P (95% CI)Difference V–P (95% CI)
Asthma symptoms26 (27)27 (27)−0.3 (−4.3 to 3.7)0.87−1.2 (−5.1 to 2.7)0.54
URT symptoms
33 (29)
29 (26)
3.8 (−0.3 to 7.9)
2.6 (−1.3 to 6.5)

*Adjusted in multiple regression analysis for confounders (see online supplement, STATISTICAL METHODS).

Definition of abbreviations: CI = confidence interval; p = placebo; URT = upper respiratory tract; V= vaccine.


The mean length of an influenza-related asthma exacerbation was 9.2 (vaccine group) and 11.2 (placebo group) and after adjustment for confounders 3.1 days longer in the placebo group (p = 0.06). There were no differences in severity (Table 3) (p = 0.11).

Respiratory Tract Episodes

From the diary data, 2,311 asthma exacerbations and 2,770 upper respiratory tract episodes were constructed (Table 3). The mean length of all asthma exacerbations (irrespective of causative agent) was 0.8 days longer in the placebo group (p = 0.01). There was no difference in number and severity of exacerbations or in number, duration, and severity of upper respiratory tract episodes. The proportion of days with respiratory symptoms did not differ between groups: 26% versus 27% for asthma symptoms and 33% versus 29% for upper respiratory tract symptoms, respectively, for the vaccine group and the placebo group (Table 3).

Subgroup analysis by vaccination history did not reveal any differences between children that had been vaccinated before and those that were vaccinated for the first time.

Medication, Health Care Use, and Absenteeism

The use of various categories of medication (maintenance, relief, oral steroids, antibiotics, etc.), analyzed both as number of doses, number of courses, and proportion of days with medication, did not differ between the groups. Unscheduled visits to family doctor or specialist and the number of absent days from school or work for participants and parents did not differ between the two groups. None of the children was admitted to hospital for airway problems.

One of the outcomes not reported in this article is quality of life during the asthma exacerbations. An article describing the translation and validation of the questionnaire used to assess quality of life has been submitted. Another article studying various secondary outcomes in more detail is in preparation.

Serologic Results

Blood samples were obtained on all three occasions from 651 children. A considerable number of children already had protective antibody levels (40 or more) before vaccination because of the previous vaccinations and/or natural infections (Table E1 in online supplement). Antibody levels and the proportion with antibody levels of at least 40 at 14–21 days after vaccination all showed differences in favor of the vaccine group. Comparing second and third samples (taken 2 to 3 weeks after vaccination and at the end of the season) in the placebo group, 17.5% of participants had at least a fourfold rise for H1N1, 4.0% for H3N2, and 0.9% for influenza B compared with 7.0%, 1.2%, and 1.8%, respectively, in the vaccine group.

Adverse Reactions

During the first 7 days after vaccination, injection site redness was reported significantly more often in the vaccine group than in the placebo group (31% vs. 8%), as were stiff or painful arms (65% vs. 25%) and myalgia (20% vs. 11%). We found no differences for reported airways symptoms during the first week or for medication use, healthcare use, and absenteeism.

This study shows that influenza vaccination was not more effective than placebo in reducing the number of asthma exacerbations caused by influenza infections in children. The duration of influenza-related asthma exacerbations was 3 days shorter in the vaccine group, but this was not statistically significant. There was no difference in severity. Regarding the other secondary endpoints, vaccinated children had shorter exacerbations (irrespective of their cause) and fewer seroconversions than the placebo group but reported more side effects. There was no difference in proportion of days with asthma symptoms.

For both seasons, the vaccine strain matched well with epidemic virus strains (14, 15). For children, the Dutch influenza bulletins reported a low number of influenza positive cases in 1999–2000 (16) and a possible epidemic increase in 2000–2001 (17), both of which are in accordance with our findings. The second season was unusual because of the relatively extensive circulation of subtype H1N1 and the low activity of subtype H3N2 (15).

We found no difference in the number of influenza-related asthma exacerbations between both groups, although the vaccine group had a significantly higher protective level of IgG. Is there a possible explanation for this lack of effect? Studies in healthy subjects have shown that parenteral inactivated influenza vaccine does not induce as good an IgA response in the mucous membranes of the respiratory tract (being the first line of defense) (18) as does priming by natural infection or intranasally administered influenza vaccine (1923). Hence, we speculate that parenteral vaccination may be relatively ineffective in preventing mucosal infection with influenza virus and subsequent asthma exacerbations.

After the 1986 study of Stenius-Aarniala and colleagues (24), this is the first randomized trial investigating parenteral influenza vaccination that takes influenza-related asthma exacerbations as an outcome (7). As the study of Stenius-Aarniala and colleagues was unsuccessful because of the extremely low influenza activity, we cannot compare our findings. Recent nonrandomized studies in children with asthma showed conflicting results (2529).

Was our study sample large enough? We were surprised by the low number of influenza-positive swabs, especially in the first season. Our choice to use pharyngeal swabs may have played a role but compared with nasopharyngeal aspirates sensitivity is only approximately 20% lower (30). By recruiting more children than originally planned in the second season, we hoped to compensate for the low incidence in the first season. The final answer to the power question is given by our results, especially the 95% confidence interval for the primary endpoint (31). The interval ranges from 34% reduction to 161% increase. This firmly excludes a 50% reduction of influenza-related asthma exacerbations by vaccination, which we considered to be the threshold for clinical relevance. Even a 35% reduction by vaccination is excluded by the confidence interval. Our finding of no difference is further corroborated by the lack of differences on almost all of our other clinical outcome measures, as shown in Table 3.

In previous studies, seroconversion throughout the influenza season was often the most important outcome measure to assess effectiveness of influenza vaccines. However, this outcome measure has serious shortcomings as a proxy measure for influenza infection (32). The most important problem is that seroconversion is more difficult or even impossible to attain in subjects with elevated titer levels at the start of the season (33).

A closer look at the difference in mean duration of all asthma exacerbations revealed that extremely long episodes (more than 3 SDs above mean) were found more often in the placebo group than in the vaccine group (n = 12 vs. n = 4). Most of these episodes began before influenza activity started in the Netherlands, which makes it likely that the difference in duration was a chance finding, not caused by the difference in treatment assignment.

Regarding adverse reactions, we found differences between groups for local symptoms (red or stiff arm) and myalgia, as reported in previous studies (33, 34). There has been some debate about exacerbations as a direct result of vaccination (34, 35). In this study, we found no differences in airway symptoms during the week after vaccination, making vaccination-induced airway symptoms unlikely.

We found relatively few influenza-related asthma exacerbations (9.1% of all calls, 1.8% of all asthma exacerbations). Given the proportion of children that showed a fourfold increase of antibodies during the season, a considerable number of influenza infections apparently did not give symptoms or gave symptoms that were not severe enough to pass our predefined threshold.

Inevitably, our episode definition, although used before (2), has arbitrary elements. Other cutoff points for the number of days between two episodes would give different results. For our primary endpoint, four children had two episodes related to the same throat swab that could be counted as one episode by stretching this criterion from 2 to 3 days. Three of these children were in the vaccine group. However, the reduction in number of episodes in the vaccine group as a consequence of changing this criterion (still negative, and the confidence interval still excluding a 50% reduction) would be balanced by an increase in length of the episodes.

In conclusion, our randomized placebo-controlled study showed no relevant effect of influenza vaccination on the number and severity of influenza-related asthma exacerbations in children with asthma. Vaccinated children tended to have shorter exacerbations throughout the season but reported more (mild) adverse effects after vaccination than children receiving placebo.

Both the limited effectiveness of influenza vaccination found in this study and the low incidence of influenza, observed in the first season, warrant additional studies to justify routine influenza vaccination of children with asthma in general practice.

The authors thank Philip Rothbarth for challenging us to design the study. They also thank the children, parents, and general practitioners who took part in the study; the research nurses and Lya Euser, who coordinated fieldwork; Frank Pistoor for his assistance on virologic and serologic matters; Kris Sieradzan and Roel Verkooyen for helping with data entry; and AstraZeneca BV, Zoetermeer, the Netherlands, for providing local anesthetic plasters.

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Correspondence and requests for reprints should be addressed to Johannes C. van der Wouden, Ph.D., Department of General Practice, Room Ff305, Erasmus MC-University Medical Center Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, the Netherlands. E-mail:


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