Understanding Vaccine Breakthrough

Early this fall, as cases of COVID-19 surged again despite available vaccines, questions about “breakthrough cases” also began to surge. The Centers for Disease Control defines a COVID-19 vaccine breakthrough as the detection of a SARS-CoV-2 infection in a person 14 days or more after they’ve completed all recommended doses of a U.S. Food and Drug Administration-authorized COVID-19 vaccine. “But isn’t the vaccine supposed to stop such infections?” people ask. We’re vexed at having to return to masking and social distancing even after receiving vaccines. Understanding how these vaccines are expected to work may help ease frustrations:

• Vaccine-induced antibodies do not prevent infection; they do play defense against the virus. When we’re exposed to airborne virus particles, antibodies in our saliva, tears and mucus attach to those particles to stop them from entering our cells. But if we’re exposed to so many virus particles that our antibodies become outnumbered, the body’s defensive line becomes overwhelmed, and the virus takes hold. This means we can’t view vaccination as a free pass to return to pre-pandemic ways.

• Viruses mutate to survive; vaccines hinder those mutations. Think of the immune system as the police force with a photograph of a criminal (thanks to a vaccine). When officers have a good photo of a criminal, they can apprehend the offender. But when the criminal changes appearances, as COVID-19 did with the Delta variant, the chances of an arrest decrease. The more vaccines limit COVID-19 spread, the less opportunity new variants have to develop (and fewer disguises means fewer bad guys roaming free).

• Vaccines train T cells to get rid of disease if an infection occurs. T cells are like the cleanup crew, killing off infected cells to rid the body of disease. This immune response is why the vaccine is so effective at staving off severe illness and death in the case of a COVID-19 infection. T cells don’t prevent infection, but they do put the virus in check if an infection takes hold.