More than 15 million Americans have received an additional dose of a COVID-19 vaccine since the U.S. began allowing boosters as a response to studies showing the efficacy of the inoculations wanes over time.
Here are five things to keep in mind about COVID-19 vaccines' waning efficacy, according to a Nov. 2 Kaiser Health News report.
- Both Pfizer and Moderna reported efficacy rates against symptomatic illness in the mid-90 percent range when their COVID-19 vaccines were granted FDA emergency use authorizations in December. When Johnson & Johnson's immunization was granted the same authorization in February, it reported 72 percent efficacy. These numbers are higher than the efficacy rates of many commonly used vaccines. For example, the flu vaccine's real-world efficacy rate in a given year is often 40 percent to 50 percent.
- When a drugmaker reports that its vaccine is 95 percent effective against COVID-19, it means a vaccinated person exposed to the virus faces just 5 percent of the risk of infection of that of an unvaccinated person. It does not guarantee that a vaccinated person will not get infected.
- Waning vaccine efficacy levels can be partly attributed to the country's loosening of social distancing requirements and other pandemic regulations. Because many Americans are finding themselves in crowds and situations they might have avoided a year ago, their virus exposure is greater, Kaiser Health News reports.
- Waning vaccine efficacy levels can also be partly attributed to virus mutations, as highly contagious variants such as delta can lower vaccine efficacy rates slightly.
- Some studies suggest time elapsed since initial COVID-19 vaccination can also contribute to waning efficacy levels. This trend is common among vaccines. For example, people must be vaccinated against tetanus every 10 years; people receive annual flu shots as well.