Discussion

Throughout the pandemic, myriad measures have been used to guide the ongoing public health response including case rates, hospitalization rates, and death rates. Because determining whether a hospitalization or death is caused by COVID-19 (either directly or indirectly) or not requires manual chart review, data obtained through simpler means, such as by counting deaths within a specified period after a positive test for SARS-CoV-2 have been used by many state and territorial health departments, but these surrogate metrics are imperfect. Our electronic tool, which gives a closer approximation to what is obtained by chart review than other commonly used measures, reveals that the proportion of deaths attributed to COVID-19 within 30 days following a positive test has decreased in both vaccinated and unvaccinated patients and that in an era with widespread immunity and effective therapies, the attributable proportion of COVID-19 deaths within a 30 day window following a diagnosis is similar between the 2 groups. These findings expand upon our prior work evaluating COVID-19 hospitalization data, in which we detected decreasing rates of severe disease following identification of medical countermeasures, which led to the addition of a new surveillance metric that was implemented statewide in Massachusetts in January 2022. ^{Reference Fillmore, La, Zheng, Doron, Do, Monach and Branch-Elliman3,Reference Doron, Monach, Brown and Branch-Elliman5} Notably, in this study, we focused only on individuals who died within 30 days of a positive test, and we did not assess the question of the effectiveness of the vaccine for preventing death.

The official national COVID-19 death toll, which is based on death certificate review, is inherently delayed. In future waves, a simple and easily implemented scoring system, such as this one, would allow state and territorial health authorities to quickly recognize whether new variants or waning population immunity are contributing to increasing attributable mortality. Additionally, integration of more accurate methods of ascertaining attributable cases may improve comparisons of mortality with other respiratory viruses, which has been a subject of debate throughout the pandemic.

As of March 2022, ∼68% of veterans engaged in VA healthcare were documented to have received at least a primary vaccination series. Thus, it is important to interpret findings considering the high levels of vaccine uptake in the VA; the greater number of deaths in the vaccinated group is due in part to larger population size. Additionally, veterans who opted not to get vaccinated are systematically different from those who received the COVID-19 vaccine. Unvaccinated patients tended to be younger and with fewer complex comorbidities; thus, they were at lower baseline risk of severe COVID-19 than vaccinated veterans, which also affected the distribution of attributable mortality. ^{Reference Bajema, Rowneki and Berry6} By September 2022, the US Centers for Disease Control and Prevention (CDC) estimated that >96% of the US population had at least some degree of immunity to SARS-CoV-2, acquired from vaccination, infection, or both. ^{Reference Jones, Manrique and Stone7} Unvaccinated veterans identified during later periods are likely to have immunity from infection that reduced their risk of severe disease during subsequent infections.

This study had several limitations. Firstly, the study was conducted in the VA; thus, women, children, and people of Asian and native American race were underrepresented in this sample. Trends in these populations may be different. Second, we were only able to assess SARS-CoV-2–positive cases that were diagnosed at or were reported to the VA. Home tests and tests conducted at outside facilities that were not reported to the VA could not be measured or evaluated for death attribution. However, cases that were diagnosed at home and not documented within healthcare systems would not be attributed to COVID-19; these limitations are similar to those encountered in other public health surveillance systems. Third, patients vaccinated within the VA may be more regular users of VA healthcare, meaning that the VA has more complete data about their medical history. Thus, some degree of misclassification of vaccinated patients as unvaccinated is possible, although the COVID-19 shared data resource does attempt to capture receipt of vaccination outside the VA if it is documented during clinical encounters.

In summary, over the course of the pandemic, COVID-19 severity, as measured by hospitalizations and deaths within 30 days of a positive test, have decreased as population immunity has increased and therapeutics have been developed. Mortality attributable to COVID-19 in patients who recently tested positive for SARS-CoV-2 has also decreased in both vaccinated and unvaccinated populations, and to a similar extent. Simple metrics for determining death attribution can be applied to improve the speed and accuracy of public health surveillance metrics.