COVID-19 and asthma
Observational studies indicated a potential protective factor of asthma for the morbidity and mortality of COVID-191,50, although conflicting data from the United States and United Kingdom (UK) suggested a higher prevalence of asthma in COVID-19 patients106. A UK study found that asthmatic patients were associated with a higher risk of COVID-19107. For allergic asthma, the protective effects have been partly attributed to the antiviral effect of eosinophils35, whose beneficial effects on COVID-19 outcomes depend on ICS108. However, whether COVID-19 patients with asthma are at higher risk of long-COVID symptoms is still unclear as there are contradictory research studies109,110.
It remains unclear whether asthma is a risk factor for the severe and worse outcome of COVID-19. However, it appears to be related to the asthma phenotype, treatment and severity111,112. Asthma was shown to be associated with an increased hospitalization risk of COVID-19 both in adults50 and in children113. Another study observed an increased hospitalization rate only in asthmatic patients needing regular ICS or regular/intermittent ICS with add-on therapy107. The hospitalization rate of allergic asthmatics was 50% lower compared to non-allergic asthmatics110. A recent meta-analysis identified preexisting asthma as a risk factor for intensive care unit (ICU) admission among COVID-19 patients114. The heterogeneity of asthma endotypes (allergic vs. nonallergic asthma) may underly the different disease course in these studies35,110. Eosinopenia was associated with worse outcomes of COVID-19, including longer duration of hospitalization, higher severity and mortality1,36,110. Dynamic monitoring of eosinophils counts in addition toother laboratory indices, such as neutrophil-to-lymphocytes ratio lymphocytopenia and D-dimer, may be used as predictive biomarkers of the outcomes of COVID-1935,36. Biologicals were associated with lower susceptibility in asthmatic patients115. Omalizumab augmented IFN-α production from plasmacytoid dendritic cells116, which may also contribute to the protecting effects of asthma against COVID-19.
A lower expression of ACE2 in bronchial epithelial or lung tissue was observed in allergic asthmatic patients106,117,118. In addition, ICS may decrease the expression of ACE2 and TMPRSS2 in bronchial epithelia of asthmatic patients59 and thus contribute to lower susceptibility to infection. The current evidence does not indicate an increased risk of long COVID-19 in asthmatic patients, although studies with more patients are warranted118. The symptoms of long COVID-19 are summarized in Table 2.
Table 2.Reported symptoms of post-acute COVID-19 syndrome or long COVID-19.