COVID-19 Vaccine Efficacy and A High-Risk Population

The past week has been rocked with increasing conversations of rising COVID-19 rates in the United States, breakthrough infections, vaccine efficacy, and the realities that COVID-19 is not going anywhere quite yet. The truth is that vaccines are an amazingly effective and life-saving tool, but don’t provide sterilizing immunity. Encouraging people to get vaccinated during a pandemic should be easy, but we’ve struggled to address core issues with hesitancy and explain the nuance of vaccination, immunity, and that even these very effective vaccines are imperfect and may require complementary strategies like masking.

Now more than ever, we’re working to understand vaccine efficacy in larger populations and what those breakthrough infections mean. How frequently do they occur? How infectious are these cases? What symptoms, if any, occur? These very questions are what we’re working through right now, especially in the face of the more transmissible Delta variant.

A new study within the Annals of Internal Medicine presented data on the Pfizer-BioNTech mRNA COVID-19 vaccine efficacy in a high-risk national population within a real-world setting. Utilizing the US Department of Veterans Affairs health care study and performing the short-term effectiveness of vaccines through a test-negative case-control study, they utilized conditional logistic regression, the researchers studied SARS-CoV-2 infection between December 2020-March 2021.

The authors reported that “Among 54,360 persons who tested positive and 54,360 propensity score–matched control participants, the median age was 61 years, 83.6% were male, and 62% were White. Median body mass index was 31 kg/m2 among those who tested positive and 30 kg/m2 among those who tested negative. Among those who tested positive, 9800 (18.0%) had been vaccinated; among those who tested negative, 17 825 (32.8%) had been vaccinated.

Overall vaccine effectiveness 7 or more days after the second dose was 97.1% (95% CI, 96.6% to 97.5%). Effectiveness was 96.2% (CI, 95.5% to 96.9%) for the Pfizer–BioNTech BNT-162b2 vaccine and 98.2% (CI, 97.5% to 98.6%) for the Moderna mRNA-1273 vaccine.”

While this was mostly a male population a short window in time, the study provided insight into a highly effective vaccine across a significant sample size. Preventing infection is a critical aspect to avoiding the spread and thus disease. While we are still working to understand efficacy in terms of breakthrough infection, such a large healthcare system study showing 95% vaccine efficacy in preventing infection is a truly impressive feat.

As the authors noted, there’s still work to be done and several limitations. “We did not have information on the reasons for testing, and we were unable to test the effect of SARS-CoV-2 variants of concern on vaccine effectiveness. Finally, we determined vaccine effectiveness over a relatively short period, and longer follow-up may be needed to determine long-term effectiveness.”

From this study, there’s great insight into vaccine efficacy with the Pfizer vaccine, but there is a desperate need to understand those breakthrough infections and ability to transmit to others. Further epidemiological studies on secondary attack rates will be critical in this age of growing cases and the Delta variant.