Discussion
The present study aimed to identify the characteristics of ILD following COVID-19 vaccination and the disproportionality between COVID-19 vaccines and ILD using the global pharmacovigilance database. We identified 679 ILD cases from VigiBase defined using MedDRA SMQ and performed disproportionality analysis. To the best of our knowledge, this is the first study to investigate the signals of disproportionate reporting of ILD associated with COVID-19 vaccines. Compared with other vaccines, no significant signal of disproportional reporting of ILD was observed for COVID-19 vaccines. These findings were consistent across several analyses conducted after considering potential biases. Moreover, the signal of disproportionality was not detected when compared with the influenza vaccine which is known to induce ILD.
In our study, reports received from European accounted for the majority of ILD cases (85.0%) following COVID-19 vaccination. In contrast to the present study, most ILD cases following COVID-19 vaccination have been reported in South-East Asia, including South Korea and Japan since Park et al reported the first ILD case following mRNA COVID-19 vaccination.15 16 18–20 Kono et al suggested that South-East Asian population should be carefully monitored since it is at a high risk of COVID-19 vaccine-related ILD.30 Among 30 cases of ILD identified following COVID-19 vaccination in the Western Pacific, which is classified as Asia by WHO, and the signal of ILD was not detected when compared with other vaccines (ROR 1.68, 95% CI 0.68 to 4.16) (table 2). However, the result of subgroup analysis according to the region should be interpreted with caution because of the small number of cases and incomplete information on ICSRs.
A previous systematic review of drug-induced ILD identified male has been as a risk factor for drug-induced ILD, especially in those treated with amiodarone, methotrexate, epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) and premetrexed.31 Males were predominant in previous case reports of ILD related to COVID-19 vaccination.15 16 18–20 However, this study observed no signal of disproportionate reporting regardless of sex (males (ROR 0.81, 95% CI 0.55 to 1.19), females (ROR 0.93, 95% CI, 0.59 to 1.46)) (online supplemental table 6). Further studies are required to identify the risk according to demographic characteristics.
We analysed the data of spontaneous reporting systems to assess signals of AE of COVID-19 vaccination. The spontaneous reporting systems have several biases due to factors that could affect reporting, which results in incorrect signal detection. These biases can be notoriety bias, information bias, selection bias and competition bias.25 32 33 We implemented different minimisation strategies against these biases. First, we designed a primary analysis to address factors that could lead to information bias by considering to be suspected, healthcare professionals and complete information on age groups and sex. Moreover, in sensitivity analysis 2, we used MedDRA SMQ with narrow and broad terms. This result was in line with the primary analysis that showed no signal of disproportionate reporting (ROR 0.77, 95% CI 0.59 to 1.00). Second, for competition bias, it is necessary to eliminate factors associated with vaccines/AEs of interest (sensitivity analyses 3, 4). Results derived from sensitivity analysis considering competition biases showed that COVID-19 vaccines had no disproportionality signal of ILD compared with the other vaccines (ROR 1.02, 95% CI 0.73 to 1.42 and ROR 0.96, 95% CI, 0.69 to 1.33, respectively). Third, in pharmacovigilance, temporal bias (the Weber effect or notoriety bias) refers to variation in the number of reports after a specific event, such as safety alerts and market authorisation. The signal of ILD was not observed when minimising temporal biases; the RORs of sensitivity analyses 5 and 6 were 0.72 (95% CI 0.52 to 0.98) and 0.75 (95% CI 0.38 to 1.48), respectively. Fourth, we defined reference groups that received influenza and other vaccines instead of all other drugs to avoid selection bias. The analysis using influenza vaccines as a positive control in the secondary analysis showed that COVID-19 vaccines emerged with no signal when compared with influenza vaccines (ROR 0.44, 95% CI 0.27 to 0.71). However, the risk–benefits of COVID-19 vaccines should be carefully assessed.
The mechanisms of COVID-19 vaccine-induced ILD are unclear. To date, both cytotoxicity and immune-mediated lung injury are considered as main mechanisms that initiate drug-induced ILD. Although it is rare, the event can be fatal and patients might require hospitalisation.34 According to previous reports, the influenza vaccination can induce ILD by increasing the levels of inflammatory cytokines.12 Several cases of COVID-19 mRNA vaccine associated ILD have been reported.15–21 Given the similar clinical characteristics with influenza vaccine-induced ILD, including onset time, chest CT findings and responsiveness to corticosteroids, it can be speculated that ILD following COVID-19 vaccination might also be due to immune-mediated pulmonary injury. These studies suggest that COVID-19 vaccination induces immune-mediated injury to the lungs through T-cells, which adopt a predominant type one phenotype in susceptible patients.17–21 35 However, further studies with a large number of ILD patients who received the COVID-19 vaccines are needed.12 15
This study has several limitations. First, selective reporting of AEs might have been compromised in the spontaneous reporting database although we strived to minimise biases. During the pandemic, the number of ICSRs following COVID-19 vaccination increased rapidly, which might have resulted in differential reporting rates and influenced parameters. We applied 1:10 exact matching to reduce the imbalance between case and non-case and performed various analyses. The results of our study did not provide exhaustivity of COVID-19 vaccine-induced ILD although it suggests focusing on the risk. Second, we analysed ICSRs without causality assessment. However, VigiBase contains essential information required for causality assessment, including age, sex, primary reporter and time-to-onset. Third, concerns on the validity of ILD in spontaneous reporting database might be raised. To overcome this limitation, we restricted physicians (77.6%), pharmacists (7.4%) and other health professionals (14.8%) as notifier types and defined ILD using MedDRA SMQs, which are validated by expert discussion. Fourth, in the present study, the majority of ILD cases following vaccination were predominantly in the European population, which may introduce bias due to population heterogeneity. Fifth, previous studies have suggested that COVID-19 infection can lead to the occurrence or exacerbation of ILD, referred to as post-COVID-19 ILD. Notably, the VigiBase we used cannot ascertain the COVID-19 infection status. Therefore, the study findings should be interpreted with caution. Finally, this study did not assess the risk of specific molecular components of vaccines. The excipients such as adjuvants, stabilisers, preservatives and trace components can cause AE following immunisation. Therefore, besides vaccines, the safety of excipients should also be evaluated. Despite these limitations, our study used a global pharmacovigilance database with over 30 million ICSRs and could offer additional hypotheses for AEs. In addition, the case/non-case approach allowed us to study rare AEs and could represent the use of drugs in real world settings.25 Since there were no population-based studies and previous case reports have included exacerbation of pre-existing ILD with death, additional safety studies are needed.