A: Two words: SILENT INFECTION.
Specifically, cruise ships that were out-to-sea during the emergence of the pandemic provided a unique setting to compute a complete count of people “at risk” of becoming infected and a complete count of people who ultimately become infected. These complete counts allowed scientists to calculate the presence of SILENT INFECTION and SILENT SPREAD, which are difficult calculations to make in real-world settings. Emerging data from these cruise ships also points to clues about individuals who switched over from having a silent course of disease to having a very worrisome one, potentially informing clinical protocols and treatments.
Cruise ships have played an outsized role in the scientific narrative about COVID. Which is perhaps a touch surprising, given that cruise ships represent a highly unique context (read: not necessarily reflective of typical day-to-day settings or broadly generalizable populations). So why are they providing some of the best information we have about the spread of COVID-19? Simply put, people on cruise ships are easy to study. And here’s the associated scientific benefit: COMPLETE DATA.
What, exactly, does complete data buy us in the context of COVID-19? It provides some of the best information we have on the extent and spread of SILENT INFECTION. In real-world settings those with silent infections may never be counted – to date, testing has been largely limited to individuals with COVID-like symptoms and/or individuals with high risk of (repeated) exposure, such as health care workers. Scientists are now fielding new studies that will provide a more refined picture of silent spread (e.g. studying sewage; testing randomly selected samples of the population), but for now, cruise ship data remain among the best we have on the issue.
Early on in the pandemic, scientists studying cruise ship settings learned that (1) The prevalence of infections that are silent is high (although an exact % is tough to nail down, ranging from approximately 20% – 80% across boats, references below); and (2) The lethality of COVID is likely much lower initially feared (again, references below).
New studies using data from those same cruise ships are now identifying predictors of “clinical deterioration” – when an individual switches from having a silent infection to having a very ”loud” one, with potentially life-threatening consequences. New work in the highly respected journal The Lancet: Infectious Disease provides clues about potential markers of clinical deterioration that can be identified by blood draws and/or lung scans taken during an individual’s “silent” infection phase. [Note that the sample sizes are small and findings require replication prior to informing clinical care.]
In conclusion, “cruise ship science” earns high marks for having COMPLETE DATA. Extending the findings to real-world settings is necessary to identify generalizability of the associated findings to real-world CONTEXTS.
“Silent infection” prevalence and lethality studies
Overview of findings from “Diamond Princess” boat (20% figure above; also discusses lower lethality than initially feared)
Findings from a scientific expedition boat (80% figure above)
For further reading:
CIDRAP at University of Minnesota’s overview of the Lancet Infectious Disease study