Dr. Ding is a member of a shrinking club of people who are pretty sure they have never been infected with SARS-CoV-2, the virus that causes Covid-19. Geneticists and immunologists are studying factors that might protect people from infection, and learning why some are predisposed to more severe Covid-19 disease.
For many, the explanation is likely that they have in fact been infected with the virus at some point without realizing it, said Susan Kline, professor of medicine at the University of Minnesota Medical School. About 40% of confirmed Covid-19 cases are asymptomatic, according to a meta-analysis published in December in the Journal of the American Medical Association.
More than two years into the pandemic, most people worldwide have likely been infected with the virus at least once, epidemiologists said. Some 58% of people in the U.S. had contracted Covid-19 through February, the Centers for Disease Control and Prevention has estimated. Since then, a persistent wave driven by offshoots of the infectious Omicron variant has kept daily known cases in the U.S. above 100,000 for weeks.
Yet some people haven’t gotten sick or tested positive.
The contribution of natural immunity should speed up the timeline for returning fully to normal. With more than 8 in 10 adults protected from either contracting or transmitting the virus, it can’t readily propagate by jumping around in the population. In public health, we call that herd immunity, defined broadly on the Johns Hopkins Covid information webpage as “when most of a population is immune.” It’s not eradication, but it’s powerful.
Without accounting for natural immunity, we are far from Anthony Fauci’s stated target of 70% to 85% of the population becoming immune through full vaccination. But the effect of natural immunity is all around us. The plummeting case numbers in late April and May weren’t the result of vaccination alone, and they came amid a loosening of both restrictions and behavior.
Researchers from the Cleveland Clinic published a study this week of 1,359 people previously infected with Covid who were unvaccinated. None of the subjects subsequently became infected, leading the researchers to conclude that “individuals who have had SARS-CoV-2 infection are unlikely to benefit from COVID-19 vaccination.”
In the largest study of its kind to date, researchers at Massachusetts General Hospital (MGH), Brigham and Women’s Hospital and the Ragon Institute of MGH, MIT and Harvard have found the new mRNA COVID-19 vaccines to be highly effective in producing antibodies against the SARS-CoV-2 virus in pregnant and lactating women. The study also demonstrated the vaccines confer protective immunity to newborns through breast milk and the placenta.
The study, published in the American Journal of Obstetrics and Gynecology (AJOG), looked at 131 women of reproductive age (84 pregnant, 31 lactating and 16 non-pregnant), all of whom received one of the two new mRNA vaccines: Pfizer/BioNTech or Moderna. The vaccine-induced titers — or antibody levels — were equivalent in all three groups. Reassuringly, side effects after vaccination were rare and comparable across the study participants.
If that’s what the vaccine trials were measuring—the height of thewall that is our immune system comparing vaccine effectiveness would make a lot of sense. Many high-profile, highly-credentialed people have been (misleadingly) describing it exactly in that manner: that if a vaccine is 95% effective, those 5% are left “unprotected.” If Moderna and Pfizer and 95% efficacious, and if Johnson and Johnson is 66%—well, that would mean that 34% of the people are left “unprotected.” right?
Wrong. To get to why that assumption is not right—and why those vaccine efficacy numbers are not the height of the wall that represents the immune system—let me first mention something important The two mRNA vaccines do appear to be spectacular, but they were tested under conditions where those pesky “variants-of-concern”—the B.117 (UK one) and B. 1.351 (South Africa) and P1–were not widespread. If tested now, under equal conditions, those numbers may be closer. Plus, Johnson & Johnson is a single-shot with a trial with a booster underway. So those efficacy numbers may well be much closer in reality than they appear from the trial results. But let’s leave that aside for a moment.