Study Contrasts COVID-19 Vaccine Hesitancy, Traffic Crash Risk

On December 13 2022, a post to Reddit’s r/science posted a link to a study examining a possible link between COVID-19 vaccine hesitancy and the likelihood of car crashes:

The appended study was published on December 2 2022 in the American Journal of Medicine (“COVID Vaccine Hesitancy and Risk of a Traffic Crash”). In its abstract, a “Background” section summarized the purpose of the research:

Coronavirus disease (COVID) vaccine hesitancy is a reflection of psychology that might also contribute to traffic safety. We tested whether COVID vaccination was associated with the risks of a traffic crash.

Authors utilized “individual electronic medical records” to inform their inquiry; a “Results” section of the abstract defined its size and scope:

A total of 11,270,763 individuals were included [in the study], of whom 16% had not received a COVID vaccine and 84% had received a COVID vaccine. The cohort accounted for 6682 traffic crashes during follow-up. Unvaccinated individuals accounted for 1682 traffic crashes (25%), equal to a 72% increased relative risk compared with those vaccinated[.] The increased traffic risks among unvaccinated individuals extended to diverse subgroups, was similar to the relative risk associated with sleep apnea, and was equal to a 48% increase after adjustment for age, sex, home location, socioeconomic status, and medical diagnoses [.] The increased risks extended across the spectrum of crash severity, appeared similar for Pfizer, Moderna, or other vaccines, and were validated in supplementary analyses of crossover cases, propensity scores, and additional controls.

The introduction described traffic accidents as a common cause of sudden death, adding that the “proximate causes of most crashes are human behaviors including speeding, inattention, tailgating, impairment, improper passing, disobeying a signal, failing to yield right-of-way, or other infractions.” It defined “vaccine hesitancy” in the context of the research as follows:

Coronavirus disease (COVID) vaccine hesitancy is defined by the World Health Organization as a delay in acceptance or refusal of vaccination against an important contagious disease despite supply (distribution), access (availability), and awareness (albeit with possible misinformation). Vaccine hesitancy or confidence is not new; for example, the original polio vaccine required multifactorial efforts, including celebrity endorsements (eg, the publicized injection for Elvis Presley in 1956). Vaccination preferences may also reflect past misadventures (eg, the ill-advised swine-flu vaccine mandate by Gerald Ford in 1976). Vaccine hesitancy in regions of wide availability, however, can be contentious due to conflicting values, fallible self-report, cognitive blind spots, or other behavioral issues … COVID vaccination is an objective, available, important, authenticated, and timely indicator of human behavior—albeit in a domain separate from motor vehicle traffic crashes.

The authors “theorized that individual adults who tend to resist public health recommendations might also neglect basic road safety guidelines.” In the abstract, they explained they identified traffic crashes requiring emergency medical care during a one-month period across 178 medical centers in Ontario, Canada; vaccination status was (as indicated above) determined using medical records.

In the “Results” section, researchers noted that of 11,270,763 adults, 9,425,473 (or 84 percent) were vaccinated against COVID-19, and 1,845,290 (or 16 percent) were not, as of July 31 2021. They added:

The largest relative differences were that those who had not received a COVID vaccine were more likely to be younger, living in a rural area, and below the middle socioeconomic quintile. Those who had not received a vaccine also were more likely to have a diagnosis of alcohol misuse or depression and less likely to have a diagnosis of sleep apnea, diabetes, cancer, or dementia. About 4% had a past COVID diagnosis, with no major imbalance between the 2 groups.

A “Results” subsection, “Traffic Crashes,” provided figures for the total number of traffic crashes in the one-month research interval (6,682). Researchers then explained:

A total of 6682 individuals required emergency care for a serious traffic crash during the subsequent month of follow-up. This rate averaged over 200 individuals per day and was comparable with population norms for high-income countries. Patients who had not received a COVID vaccine accounted for 1682 crashes (25% of total crashes), equal to an absolute risk of 912 per million. Patients who had received a COVID vaccine accounted for 5000 crashes (75% of total crashes), equal to an absolute risk of 530 per million. The difference corresponded to a relative risk of 1.72 for patients who had not received the COVID vaccine[.] The risk of a traffic crash was proportional with time for both groups[.]

A “Discussion” section at the end of the study included observations made by the researchers, as well as limitations they identified in the data set. The authors proposed possible reasons for both behaviors (vaccine hesitancy and “risky driving,”) and explained:

A limitation of our study is that correlation does not mean causality because our data do not explore potential causes of vaccine hesitancy or risky driving. One possibility relates to a distrust of government or belief in freedom that contributes to both vaccination preferences and increased traffic risks. A different explanation might be misconceptions of everyday risks, faith in natural protection, antipathy toward regulation, chronic poverty, exposure to misinformation, insufficient resources, or other personal beliefs. Alternative factors could include political identity, negative past experiences, limited health literacy, or social networks that lead to misgivings around public health guidelines. These subjective unknowns remain topics for more research.

Another limitation of our study is the lack of direct data on driving exposure in different groups. A 100% increase in driving distance, however, is unlikely to explain the magnitude of traffic risks observed in this study. A difference in driving distance would also not explain why the increased risks extended to pedestrians, why the increased risks were not lower in urban locations, and why the increased risks were not higher on weekends (when discretionary driving is common). To be sure, physical factors such as vehicle speed and distance are controlled by the driver and part of the mechanism that ultimately results in a traffic crash. These physical unknowns do not change the importance of our study for estimating prognosis.

Researchers also referenced a prior survey, which they said showed that individuals who had not received an influenza vaccination were 15 percent more likely to report that they took risks while driving.

Finally, a section in “Appendix,” “Accounting for Later Vaccinations,” included information about vaccination rates in the initially unvaccinated group and their prevalence in crashes during the one-month period:

The study examined vaccination status based on records on July 31, 2021 and did not include possible later vaccination that might have eventually occurred. In turn, we retrieved information on these subsequent vaccinations and considered extreme assumptions to examine how results might change based on the crossover cases. Specifically, we found 219,740 individuals who were eventually vaccinated from the cohort of 1,8450,290 who had been classified as unvaccinated. These individuals accounted for 155 total traffic crashes during follow-up.

The researchers found that “COVID vaccine hesitancy is associated with an increased risk of a traffic crash,” adding that a “direct” link between the two was “unlikely.” They posited that “diverse psychological factors” were a likelier factor, postulating that “vaccine willingness and driving safety” were behaviors which “entail inconveniences advocated by authorities to protect the community.”

While correlation is not to be confused with causation, the results of this study appears to validate the existence of — and may help identify — specific social and behavior pathways along which disinformation is spread.