The vaccines were tested at different times and in different places. The intensity of the epidemic and measures to mitigate it, such as mask-wearing, may contribute to differences in efficacy estimates between countries.
SARS-CoV-2 has mutated over time, generating variants that appear to be more dangerous. So, in general, the first vaccines to prove effective likely faced fewer of these viral strains than subsequent ones have.
Vaccines take time to work, and the time periods during which efficacy was measured in clinical trials differ across studies.
Some trials may exclude participants with pre-existing conditions that could affect their response, while another trial might include such people. For example, Novavax reported a modest decline in efficacy in South Africa when HIV-infected individuals were included in the analysis.
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