Tuesday September 8 2009
Some studies suggest nursery can build a resistance to asthma
The Daily Telegraph has reported that “taking children to daycare and nursery does not help them to build immunity against asthma and allergies”. This conclusion comes from a study that followed 4,000 children from before birth to age eight. Although it found that children who attended daycare early (before age two) had more airway-related symptoms up to the age of four, these symptoms decreased beyond this age, with daycare attendance having neither a protective or harmful effect on asthma symptoms at age eight.
This study had a number of strengths including its large size, prospective design, relatively long period of follow-up and retention of a large proportion of the participants over several years of study. However, although the study did assess some objective measures of airway function, other outcomes included parental reports of wheeze and other asthma symptoms such as shortness of breath.
Asthma is notoriously difficult to diagnose in young childhood and such symptoms can be caused by infection and alone do not indicate a definite clinical diagnosis of asthma. The study followed children up only to age eight and it is not known whether the asthma symptoms observed would progress to definite asthma in later childhood and adolescence.
Where did the story come from?
This research was conducted by Dr Daan Caudri and colleagues from Erasmus University and other research centres in the Netherlands. No sources of funding were reported by the study and the authors say they had no conflicts of interest.
The study was published in the peer-reviewed American Journal of Respiratory and Critical Care Medicine.
What kind of scientific study was this?
This was a prospective cohort study. It looked at factors that might increase or decrease the risk of children developing asthma or allergies as part of the Prevention and Incidence of Asthma and Mite Allergy birth cohort study. In this current piece of research the authors were investigating the affects that daycare attendance (such as in a nursery) might play.
Some researchers have suggested that although daycare attendance increases the risk of infections, early exposure may reduce the risk of developing asthma and allergies in the long-term, possibly by influencing the way the immune system develops. This theory is known as the hygiene hypothesis.
The researchers enrolled 3,963 children born during 1996 and 1997. Their mothers had filled out questionnaires during their pregnancies, then when the children were aged three months, 12 months, and then annually up to the age of eight years. These surveys included questions about the children’s airway-related symptoms (such as wheezing) from age one. From age two they also featured questions about shortness of breath and prescription of inhaled steroids. There were further questions on the presence of older siblings and about daycare attendance (defined as at least four hours a week in a professional daycare institution where they had contact with other children).
When the children were eight years old, 3,518 of them were invited to provide blood samples for testing for allergic sensitisation to common allergens (house dust mite, cats, dogs, certain pollens, and fungi). All 988 children whose mothers had allergies were invited for a medical examination, as were 566 randomly-selected children whose mothers did not have allergies. This examination included a test of how well the children’s airways and lungs functioned (known as spirometry) and a test that helps to diagnose whether or not a person has asthma, called a methacholine challenge test.
The researchers defined asthma symptoms as at least one attack of wheeze or at least one attack of shortness of breath or a prescription of inhaled steroids (after age of two years) or a combination of these. Allergic asthma was defined as asthma symptoms plus sensitisation to at least one airborne allergen. Parents reported serious infections of the respiratory tract in the past year, with three or more in this time considered to be frequent.
Children were divided into three groups: those who attended daycare early (before age two), those who attended late (from age two to four), and those who did not attend daycare. The researchers then compared the outcomes of these three groups. The researchers’ analysis took into account maternal allergies or asthma, maternal age, smoking during pregnancy, parental education, single parenthood, gestational age and birth weight of the child, breastfeeding, child gender, tobacco smoke exposure at home, type of home location (urbanisation), presence of pets and siblings.
After eight years, 92% of children were still taking part in the study, with full information on exposures available for 1,643 of the children. In just over a third of those initially enrolled (36% or 1,445 children) at least one questionnaire was missing, and these children were more likely to have a mother with allergies or asthma, a mother with a low level of education, and were less likely to attend daycare before the age of five years.
Data on allergic responses was obtained in 49% of children asked to attend for blood tests and data on airway responsiveness was available on 60% of the children. There was no association between response rates and the researchers used statistical methods to create estimates of missing data.
What were the results of the study?
At eight years of age, 15% of children had at least one of the three asthma symptoms (wheezing, shortness of breath or use of inhaled steroids).
Children who attended daycare before age two were twice as likely to experience wheezing before age one compared with children who did not attend daycare (odds ratio [OR] 1.89, 95% confidence interval [CI] 1.50 to 2.39). However, by age five and up to the age of eight there was no significant difference between these groups in wheezing.
The combined outcome of asthma symptoms (wheezing, shortness of breath or prescription of inhaled steroids), was assessed from ages three to eight years. No significant difference was found in asthma symptoms between those attending daycare early, attending late or not attending daycare at all.
Using a stricter definition of asthma symptoms ("frequent wheezing four or more times per year" and a "doctor’s diagnosis of asthma with asthma symptoms in the past year") or early daycare attendance (defined as attendance before the age of six months), still showed no protective effect of daycare attendance on outcome at age eight.
Children with older siblings had more wheezing by age one than children who did not have older siblings (OR 2.15, 95% CI 1.81 to 2.56). However, this association decreased with increasing age and disappeared at age eight. The presence of older siblings did not reduce the risk of wheeze, inhaled steroid prescriptions or asthma symptoms at any age.
Children who attended early daycare and had older siblings had over four times the risk of frequent respiratory infections and more than twice the risk of wheezing in the first year compared with children without older siblings who did not attend daycare. However, there was no difference in wheeze, inhaled steroid prescription, or asthma symptoms between these groups at age eight.
What interpretations did the researchers draw from these results?
The researchers concluded that they “found no evidence for a protective or harmful effect of daycare on the development of asthma symptoms” at eight years old. They suggest that early daycare “should not be promoted for reasons of preventing asthma and allergy”.
What does the NHS Knowledge Service make of this study?
This study has a number of strengths, including its large size, prospective design, relatively long period of follow-up, retention of a large proportion of the participants over follow-up, and use of a number of objective measures of airway function.
The criteria used for asthma symptom diagnosis in this study is one of the aspects worth considering when interpreting its results, as asthma is notoriously difficult to diagnose in young childhood. Often a nocturnal cough can be the only symptom. The authors looked at various outcomes and asthma symptoms were considered to include at least one attack of wheeze or at least one attack of shortness of breath or a prescription of inhaled steroids (after two years of age), or a combination of these. Symptoms of wheeze or shortness of breath may be caused by infection and alone do not indicate a definite clinical diagnosis of asthma.
Although the authors report carrying out a sensitivity analysis that used stricter criteria it is not clear for what proportion of the children they had this more definite diagnostic information available. The authors do note that there is no ‘gold standard’ way of diagnosing asthma in children. However, they did find similar results when they used different definitions.
Other points to consider when interpreting this study:
- Asthma symptoms were reported by parents and this may have led to some inaccuracies. However, the authors thought that this was unlikely to bias their results as it was unlikely that these inaccuracies would affect one group of children (daycare or no daycare) more than the other.
- This study has looked at asthma symptoms up to the age of eight years. It is not known whether asthma symptoms would progress to definite asthma in later childhood and adolescence. Future studies will be needed to determine whether these findings also apply to older children.
- The authors report that the average number of children in a daycare group in the Netherlands is 10. Classes of different sizes might have different effects.
- A number of children did not have the objective tests of airway function and allergic response, and some had missing questionnaires. This could have affected the results.
Asthma has a number of different causes, including hereditary factors, exposure to allergens such as pets and dust mites, infections and environmental factors including household smoke and other irritants. Whether or not a child is sent to daycare is likely to, at most, have only a limited effect on whether the child goes on to develop asthma.