The effect of different SARS-CoV-2 variants.

To the Editor

We read with interest the retrospective cohort study by Wang et al., which assessed the association between COVID‐19 vaccination and the risk and the clinical outcomes of SARS‐CoV‐2 infection in a large population of older adults between December 2020 and August 2021. They found that compared to those without dementia, patients with dementia were associated increased risk of breakthrough infection in vaccinated populations. However, we have one serious concern about their findings.

Although Wang et al. using the propensity score matching method to balance the demographic features of patients with and without dementia, the time variable was neglected. The epidemiology of SARS‐CoV‐2 had rapid evolutionary change since its first outbreak in Wuhan, China. In September 2020, the emergence of the first variants of concern (VOCs)—the alpha strain was identified in UK which was followed by several major concerns, including beta strain in South Africa in later September, gamma strain in Brazil in November 2020, delta strain in India in December 2020, omicron strain in South Africa in November 2021, that developed with time. , However, they have had various impacts on transmissibility, immunity, and severity. The alpha strain could increase transmissibility and severity, but it had drastically reduced circulation following the emergence of delta strain and vaccine induced immunity. By contrast, although omicron strain can increase transmissibility its association with COVID‐19 severity is reduced, which can be associated with a better clinical outcome, such as a lower risk of hospitalization and mortality. Moreover, the effect of COVID‐19 vaccine on different VOCs was not the same. During the study period, all these VOCs except omicron variant would be the major concerns. To overcome this confounding effect caused by SARS‐CoV‐2 variants, it is better to match the index date using the date of full vaccination.

CONFLICTS OF INTEREST

Author disclosures are available in the supporting information.

Supporting Information

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