ABSTRACT
Background: By December 2021, administration of the third dose of COVID-19 vaccinations coincided with the spread of the Omicron variant in Europe. Questions had been raised on protection against infection conferred by previous vaccination and/or infection. Method: Our study population (n = 252,433) included all those in the COVID-19 vaccination registry in Malta who were vaccinated with only 2 doses equivalent by 15th December 2021, and not vaccinated with an additional dose by 8th March 2022, and alive by 8th March 2022. Data were then matched with the national testing database (all RT-PCR/Rapid Diagnostic Tests - RDT tests) until 8th March. We collected vaccination status, vaccine brand, vaccination date, infection history, and age. Using logistic regression, we examined different combinations of vaccine dose, prior infection status and time, and the odds of infection during the Omicron period (December 15th, 2021 onwards). Results: Results found that participants infected with Sars-Cov-2 prior to the Omicron wave had a significantly lower odds of being infected with the Omicron variant. Additionally, the more recent the infection and the more recent the vaccination, the lower the odds of infection. Receiving a third dose within 20 weeks of the start of the Omicron wave in Malta offered similar odds of infection as receiving a second dose within the same period. Conclusion: Time since vaccination is a strong determinant factor against infection, as was previous infection status and the number of doses taken. This finding reinforces the importance of future booster dose provision especially to vulnerable populations.
Subject(s)
COVID-19ABSTRACT
Background: Underlying conditions have been found to be associated with severe COVID-19 outcomes, such as hospitalisation and death. This study aimed to estimate age-specific adjusted relative and absolute effects of individual underlying conditions on hospitalisation, death and in-hospital death among COVID-19 cases.Methods: We analysed case-based COVID-19 data submitted to The European Surveillance System (TESSy) between 2 June and 13 December 2020 by nine European countries. We individually assessed the association between 11 underlying conditions with hospitalisation, death and in-hospital death. Two additional categorical exposures were created: number of underlying conditions (1,2, ≥ 3) and the presence of any underlying condition (≥ 1). Adjusted ORs (aOR) for the association between each exposure condition and outcome were estimated using two multivariable logistic regression models: 1) an age-adjusted model and 2) an age-interaction model (exposure condition*age). All models were adjusted for sex, reporting period (June-September; October-December) and reporting country. From the age-interaction model we estimated the predicted probability of the three outcomes for each level of condition and age-group, marginalised over the levels of each covariable.Findings: After controlling for age, sex, reporting period and reporting country in the age-adjusted models, cases with cancer, cardiac disorder, diabetes, immune deficiency disorder, kidney disease, liver disease, lung disease, neurological disorders, obesity, any underlying condition or up to three or more conditions were between 1·5 and 5·6 times more likely to be hospitalised or die than cases with no underlying condition. Asthma was associated with increased overall risk of hospitalisation, not death. Age was an important modifier of these associations, with an age interaction present in the majority of models. For all outcomes, age-specific aOR in the age-interaction models tended to decrease with increasing age, whereas predicted probabilities of the outcome increased with age. For instance, individuals aged <20 years with any underlying condition were significantly more likely to be hospitalised (aOR: 5·16, 95%CI: 4·42 - 6·02) and die (aOR: 33·77, 95%CI: 12·57 - 90·75) compared to same-aged individuals without condition. The aOR fell to 1·77 (95%CI: 1·71 - 1·83) and 1·61 (95%CI: 1·55 - 1·68) respectively in individuals 80 years and older. Conversely, the predicted probabilities of hospitalisation and death among cases aged <20 years were 5·69% (95%CI: 4·97 - 6·51) and 0.15% (95%CI: 0·08 - 0·31), respectively, while they were 44·55% (95%CI: 43·68 - 45·43) and 16·31% (95%CI: 15·44 - 17·21), respectively for individuals aged 80 years and older. For some conditions, the probability of the outcome was at least as high in younger individuals with the condition as older cases without the condition.Interpretation: Several underlying conditions were found to have a significant independent effect on severe COVID-19 outcomes. Age is an important effect modifier in these associations. Interpretation of the results in this study is facilitated by considering together the estimates of relative (aOR) and absolute (predicted probabilities) effects that are presented. The presence of underlying conditions tended to have a larger relative effect in the young than the old, but the absolute probability of being hospitalised or dying increased with age. The finding that for some conditions, a younger person may have the same or even higher probability of severe outcome than an older person without it, has relevance for age and risk-factor based prioritisation of vaccination, particularly in the young.Funding Information: This study was funded through ECDC internal funding.Declaration of Interests: None to declare.