Introduction
The prevalence of asthma is frequently studied and a consistent pattern
of a worldwide increase during the latter half of the 20th century has
been reported both among children and adults, although the increase may
have leveled off in some areas [1,2]. The reason for the increase is
unclear, but changes in lifestyle, environments, and rapid urbanization
may have contributed [1,3-5]. Prevalence is mainly a function of
incidence and remission. In adulthood, asthma is regarded as a chronic
disease due to low remission [6,7], why the prevalence among adults
is a result of the high incidence in childhood and adolescence
[8-11] with persistence into adulthood, but also due to the
relatively high incidence in adulthood [4,12-24]. However, incidence
studies of asthma among adults are scarce, particularly population-based
studies where cohorts are followed prospectively.
Comparing incidence rates of asthma is complex as the results are highly
sensitive to the age of the study population and the methods used.
Reported incidence estimates vary substantially but are higher among
children and teenagers, 10-30/1000/year [8-11,20], than among
adults, 0.2-5/1000/year [4,12-24]. Results from retrospective
[11,13,22,24,25] and register studies [13] tend to generate
lower incidence rates than prospective studies [11-21,25]. The
definitions of both the outcome and the population at risk significantly
influences the incidence rate [12,15,18,19], which highlights the
importance of using the same methods and definitions to enable
comparisons of results from different studies and over time. Three
studies in Sweden or the neighboring countries, whereof one in the same
area as the current study, based on identical methods reported similar
incidence rates of physician-diagnosed asthma among adults from the
1980’s up until 2006, 2-3/1000/year [15,16,18], but it is unknown if
the incidence has changed thereafter. As the prevalence of asthma in
adults has increased in Sweden during the last decades [2,26] we
hypothesized that the incidence of asthma in adulthood still is high or
even has increased during the same period. Thus, the main aim of the
current study was to estimate the incidence of asthma among adults
during the period 1996 to 2016 by surveys of two cohorts each followed
for ten years, one from 1996 to 2006 and the other from 2006 to 2016. A
further aim was to identify risk factors for adult onset asthma.