In a paper in the October 2020 issue of the
American Journal of Preventive Medicine, researchers
modeled what a COVID-19 vaccine with varying types of protection could mean.
They found that if a vaccine protects 80 percent of those immunized and 75 percent of the population is vaccinated, it could largely end an epidemic without other measures such as social distancing. “Otherwise, you won’t be able to rely on the vaccine to return us to ‘normal,’” says
Bruce Y. Lee, a co-author of the paper and a professor at the CUNY Graduate School of Public Health and Health Policy. That is, if the vaccine only prevents disease or reduces viral shedding rather than eliminating it, additional public health measures may still be necessary. Even so, Lee stressed that a widespread nonsterilizing vaccine could still reduce burden on the health care system and save lives.
Influenza may provide the best blueprint of what to expect going forward. The
most common flu vaccine—the inactivated virus—is not “truly sterilizing because it doesn’t generate local immune response in the respiratory tract,” Crowcroft says. This fact, coupled with low immunization rates (often
shy of 50 percent among adults) and the influenza virus’s ability to
infect and move between multiple species, enables it to constantly change in ways that make it hard for our immune system to recognize. Still, depending on the year, flu vaccines have been shown to
reduce hospitalizations among older adults by an estimated 40 percent and intensive care admissions of all adults by as much as 82 percent.
Research on
seasonal coronaviruses suggests that SARS-CoV-2 could similarly evolve to evade our immune systems and vaccination efforts, though probably at a slower pace. And data remain mixed on the relationship between
symptoms,
viral load and
infectiousness.
But ample precedent points to vaccines driving successful containment of infectious diseases even when they do not provide perfectly sterilizing immunity. “Measles, diphtheria, pertussis, polio, hepatitis B—these are all epidemic-prone diseases,” Crowcroft says. “They show that we don’t need 100 percent effectiveness at reducing transmission, or 100 percent coverage or 100 percent effectiveness against disease to triumph over infectious diseases.”