Researchers have calculated a key number—the threshold of protection for other vaccines. Covid-19’s is still a mystery.
Why don’t Covid-19 vaccinations last longer?
Measles shots are good for life, chickenpox immunizations protect for 10 to 20 years, and tetanus jabs last a decade or more. But U.S. officials are weighing whether to authorize Covid-19 boosters for vaccinated adults as soon as six months after the initial inoculation.
The goal of a vaccine is to provide the protection afforded by natural infection, but without the risk of serious illness or death.“A really good vaccine makes it so someone does not get infected even if they are exposed to the virus,” said Rustom Antia, a biology professor at Emory University who studies immune responses. “But not all vaccines are ideal.”
The three tiers of defense, he said, include full protection against infection and transmission; protection against serious illness and transmission; or protection against serious illness only.
The effectiveness depends on the magnitude of the immune response a vaccine induces, how fast the resulting antibodies decay, whether the virus or bacteria tend to mutate, and the location of the infection.
The threshold of protection is the level of immunity that’s sufficient to keep from getting sick. For every bug, it’s different, and even how it’s determined varies.
Sources: San Francisco Department of Public Health (hepatitis A); National Institutes of Health (human papillomavirus); Centers for Disease Control and Prevention (tetanus, typhoid, influenza, Covid-19)
“Basically, it’s levels of antibodies or neutralizing antibodies per milliliter of blood,” said Mark Slifka, a professor at Oregon Health & Science University.
(T-cells also contribute to protection, but antibodies are easier to measure.)