COVID-19 immunity after Pfizer and Moderna mRNA vaccines begins to wane around three months after the second dose and accelerates at four months, according to a US study of veterans during a Delta wave.

The researchers looked at data from nearly 15,000 male veterans over 65 to see how well the vaccines were protecting them from any infection before and during the spread of Delta.

The team used electronic medical record data from the Veterans Health Administration Corporate Data Warehouse. The study separated the observation period into three periods based on CDC SARS-CoV-2 variant tracking data: pre-Delta (before May 2021), rising Delta (May and June 2021), and high Delta (July through September 2021), when the Delta variant accounted for more than 70% of new infections in the US.

In this study, 14 238 male veterans aged 65 years or older with a positive SARS-CoV-2 test result were included included as cases, while 56 952 veterans with a negative test result were included as controls.

The estimated efficiency of the pre-Delta mRNA vaccine against any SARS-CoV-2 infection was 94.5 percent (95 percent confidence interval [CI], 90.7-96.7) in the first month following completion of vaccination, but fell to 87.9 percent (95 percent CI, 85.9-89.5) by month 3.

During the high-Delta phase, vaccination effectiveness was estimated to be 62.0 percent (95 percent confidence interval [CI], 45.6-73.5) in the first month and fell to 57.8 percent (95 percent CI, 52.5-62.5) by month 3, consistent with the pre-Delta period.

After month 4, the decline in vaccine effectiveness intensified, reaching a low of around 20 percent in months 5 through 7.

While the vaccines were 94.5 per cent effective at preventing infection before Delta, decreasing to 87.9per cent after three months, they were 62 per cent effective during Delta, decreasing to 57.8 per cent after three months.

The researchers say the fact that the study group was entirely older men could impact the results of this study.

Source: 10.1001/jamanetworkopen.2021.38975

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