The sought-after state of herd immunity — in which widespread outbreaks are prevented because enough people in a community are immune to a disease — is complicated by open questions about the effectiveness of a future vaccine and how COVID-19 spreads.

Why it matters: Unless a sufficient level of immunity is achieved in the population, the coronavirus could circulate indefinitely and potentially flare up as future outbreaks.

Where it stands: The magic number often cited is a minimum of 60% of the population would need to have immunity.

• Right now, antibody studies indicate the world isn’t close to that threshold, the NYT reports.
• In hard-hit New York, for example, a recent study found 19.9% of people tested have antibodies to SARS-CoV-2. (Though even that is debated by researchers.)
• And in Sweden, which took (controversial) relaxed measures in controlling the coronavirus, just 7.3% of Stockholm’s population developed antibodies by April.

The catch: Antibodies are only meaningful to herd immunity if they provide lasting protection from a virus after someone is infected or vaccinated. But with the novel coronavirus, it’s unknown how much immunity a person has after being infected — and for how long.

• While not about SARS-CoV-2, a recent study, yet to be peer-reviewed, measured the level of antibodies to four other seasonal human coronaviruses in 10 people over a period of 35 years and found the antibody levels dropped as soon as six months after infection.
• They also found people were frequently reinfected with the same virus after 12 months.
• “An alarmingly short duration of protective immunity to coronaviruses was found,” the authors from Amsterdam UMC wrote.

“There is not a consensus for what herd immunity looks like for this disease,” says Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine.

• Sarah Cobey, associate professor at the University of Chicago, agrees and tells Axios herd immunity may require more than 60%.
• But some researchers have suggested that because different populations seem to be more susceptible to the disease, herd immunity may be achieved at lower levels.

“The biggest obstacle right now is that we have no vaccine. … Then it will be distribution to the world’s population. And, after that, you’d really be looking at [whether] the uptake is high enough or are there pockets of people who don’t want to get the vaccine,” Adalja says.

• In order for a vaccine to bring herd immunity, both its efficacy and the percentage of people immunized would have to be high, Hotez says.
• He and his colleagues estimate a COVID-19 vaccine would have to have an efficacy of 70% to prevent or extinguish an epidemic without social distancing and other measures, according to a recent preprint study.

But vaccinating large swathes of the population may be challenging.

• Recent polls find between about 50% and 70% of Americans say they would get a COVID-19 vaccine if it became available.
• Still, Adalja points out that as opposed to a highly contagious disease like the measles — which requires over 90% herd immunity — small pockets of anti-vax resistance likely won’t stymie efforts to contain COVID-19.

Yes, but: Concern about anti-vaxxers is growing. Joe Smyser, CEO of the Public Good Projects, tracks the anti-vaccination movement and recently launched a digital tool called Project VCTR.

• In a press conference last week, he said anti-vax messages on social media have tripled since the pandemic started.

What to watch: Over time, the virus could settle into a regular seasonal pattern not so different from coronaviruses that cause the common cold, recent research by Harvard epidemiologist Marc Lipsitch suggests.

• In the near term, that could set up the virus for future outbreaks, like those seen with measles, or, if it evolves, the emergence of another strain of the virus that humans have never encountered.
• Even with herd immunity, “there are going to be pockets of people who have lower [immunity] coverage, and you still have the opportunity for there to be small outbreaks. So it doesn’t hurt to be vaccinating well above the minimum,” Cobey says.
• Eventually, it’s possible that as at least part of the population becomes immune, the effects of the virus could become milder, the Washington Post reports.