To prevent the twin-demic, where the COVID-19 pandemic and the 2020-21 influenza epidemic overlap, Korea started a national influenza vaccination program on September 25, 2020. Although the program has been conducted annually, protocol violations during the vaccine transport have been reported this year which led to public anxiety and the Korea Centers for Disease Control and Prevention (KCDC) decided to stop vaccination for 2 weeks.1 During the period, the KCDC reviewed and investigated the quality and safety of the vaccines. Recently, multiple cases of death after vaccination are being reported through mass media, which brought about chaos in the field and public doubts on the safety of vaccination. Distrust in the national vaccine program can cause a fundamental problem in public health, and concerns are growing as it can lead to campaigns against vaccines as in the United States and Europe.
On October 22, 2020, the media reported a total of 28 deaths after vaccinations.2 At present, 12 vaccines (2 imported, 10 domestic products) are supplied for the national vaccine program,3 but the deaths are not limited to any specific product and geographic localities.
However, haste suggestion of an association between those deaths and vaccination has a logical pitfall. The real concern of the public is potential death resulting from the side effects of the vaccine, but what we are observing so far is mere that the deceased people were vaccinated shortly before death. This is a typical example of a conditional probability, which does not imply any association or causation by itself.
This is proved by simple reasoning. About 300,000 deaths occur annually in Korea, and the average daily death toll rises slightly during winter. Thus, it can be estimated that there will be about 1,000 deaths every day around October. Assuming that the national influenza vaccination rate is about 50% and the vaccination period is about 2 months, about 1% of the total population will be vaccinated every day during the vaccination period. Without consideration of age and sex, as much as 1% of 1,000 deaths per day in October (about 10 deaths) would occur as deaths within a day since vaccination. Because about 10% of the causes of death in Korea are unknown, the cause of death in 10% among the 10 post-vaccination deaths every day will be unknown. In other words, it can be seen that death after vaccination at the currently reported frequency is not unusual, even with a simple estimation.
What makes this problem more complicated is a recall bias. Recall bias is one of the most important problems in epidemiological studies, and it refers to the phenomenon of reinforcing memories and statements about moral reproof or memorable prior events when certain events occur. Vaccination by itself can be the subject of recall bias, as it might be perceived as a significant event for a person who has been healthy and had no recent unusual events.
On October 21, 2020, the KCDC announced the results of a preliminary epidemiological investigation. Let's look at the possible side effect scenarios based on the report.