By: Christopher Sirota, CPCU
In the world of communicable diseases, no news is definitely not always good news: symptom-less transmission reportedly happens and can contribute significantly to viral spread.
What is Considered Symptom-Less?
Asymptomatic infected individuals can transmit a disease without experiencing any symptoms, similarly presymptomatic infected individuals can transmit a disease but eventually do develop symptoms days or weeks later.
What Percentage of Cases are Likely Due to Symptom-less Transmission?
Many studies have sought to quantify the contribution of symptom-less transmission during the COVID-19 pandemic. Here are a few:
- A January 2021 study published in JAMA estimated that asymptomatic and presymptomatic transmission combined likely accounts for more than half of all transmissions.
- Another study, published in the Proceedings of the National Academies of Science in March 2021, evaluated testing data from New York City, and also concluded about half of the transmissions are symptom-less, and, as such, recommended a greater emphasis on asymptomatic testing and contact tracing to better contain the spread of the virus.
- A Duke University study, published by the Centers for Disease Control and Prevention (CDC) in November 2020, similarly found that about half of COVID-19 infections on campus were asymptomatic, and suggested asymptomatic testing could help mitigate viral spread.
- A related January 2021 study published in the Annals of Internal Medicine estimates about one third of people infected with the COVID-19 virus do not develop symptoms.
For comparison, a 2014 study published in The Lancet estimated that about three-quarters of influenza infections were asymptomatic.
Vaccines May Help
Some recent studies seem to indicate one type of vaccine, messenger RNA (mRNA), so far, may be effective in helping to reduce asymptomatic transmission.
Science Daily has reported that researchers at the Mayo Clinic evaluated a cohort of 39,000 patients who were screened for COVID-19 as a hospital pre-procedure; the cohort included 3,000 patients who had at least one dose of an mRNA COVID-19 vaccine (Pfizer-BioNTech or Moderna). The researchers concluded that after one dose, patients were 72% less likely to test positive for COVID-19, compared to an unvaccinated group; overall the researchers found that after a second dose, adjusting for other factors, those patients were 80% less likely. The researchers interpreted their findings as follows, per the article: "[…] these findings underscore the efficacy of messenger RNA vaccines for COVID-19 to significantly limit the spread of COVID-19 by people with no symptoms who may unknowingly spread the infection to others."
CNBC has also reported on a study of the effectiveness an mRNA vaccine on asymptomatic transmission. According to CNBC, a study of the effectiveness of the Pfizer mRNA vaccine in Israel has revealed a 97% effectiveness against symptomatic cases, and 94% effectiveness against asymptomatic cases. The article notes the analysis took place from January to March 2021 during which the dominant strain was the UK B.1.1.7 variant.