The premise is that if COVID-19 infection protects at least some people from reinfection for some time, vaccination (because vaccines rarely out-perform the body’s own immune reaction), proof of immunity that could allow freer travel, and resumption of many stalled economic and social activities may be possible.

Meanwhile, the US is continuing to lose the battle against the virus. COVID-19 is spreading in most of the US at rates too high for effective contact tracing, too high for safe in-person schooling, and too high for robust economic recovery.

There are likely between 100,000 to 500,000 new infections a day. The Centers for Disease Control and Prevention now estimates that between 30 and 60 million people in the US have been infected since the outbreak began—10-20% of the population. COVID-19 is now the third leading cause of death in the U.S.

But can herd immunity result from 20-30% infection, as some misguided academics suggested recently—far lower than the usual estimate of 60-80% infection? Don’t count on it. Some communities already have a 50% infection rate or higher — this would be impossible if herd immunity kicked in at a 20-30% infection rate!

It’s not either-or

Like other oversimplifications — open or closed businesses, droplets vs. aerosols — herd immunity is a continuum, not an on-off phenomenon. When more people are immune, infection spreads more slowly. But because spread is uneven, many communities remain vulnerable even if a high proportion of the population has been infected. Herd immunity, then, is not protection for all—even if lots of people have been infected.

Recent laboratory, animal, and human investigations give hope that natural immunity to COVID-19 does occur following infection. In one of the most interesting studies, researchers this week released an intriguing new report, which has not yet been peer-reviewed.

During a large COVID-19 outbreak on a fishing ship, three people with a particular type of antibodies known as neutralizing antibodies (because they neutralize the virus in a test tube) don’t appear to have gotten infected when nearly everyone else on the ship did. Before the ship’s departure, these three people and three others had tested positive for antibodies.

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The other three had antibody tests that were either falsely positive or weakly positive (there’s no way to know the difference at this time), perhaps from mild prior COVID-19 infection; they did not have neutralizing antibodies.

Two of these three became infected on the ship. In contrast, two of the three people who did have neutralizing antibodies did not become infected. The third had low levels of PCR positivity, which may have reflected either low-level reinfection or persistent PCR positivity—a phenomenon described in some people after infection with the virus.

Long-lasting immunity?

It’s not enough to bet your life on, but it’s enough to be optimistic about the potential for natural immunity after exposure to COVID-19; more severe illness seems to correlate with more robust immune protection. The fishing ship outbreak investigation didn’t provide information on symptomatic illness, if any, of the 6 people who were antibody positive before departure.

Next, we have to figure out how long immunity from neutralizing antibodies lasts, how complete it is, and what proportion of infected people are protected.

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Now the peril. Our immune reaction isn’t all good. Developing immunity — either through infection or vaccination — may result in severe illness. This is why some people get so sick with COVID-19, and likely why dexamethasone, a medication that weakens the immune system, helps some severely ill patients.

And it’s a concern with vaccination, which could potentially harm a small proportion of patients because of immune over-reaction — either from the vaccination itself, or from exposure to the virus after vaccine-induced immunity. This is being studied carefully in the vaccines being developed and will need to be tracked whenever they are given.

Getting careless

The other peril of immunity is overconfidence, which may lead individuals and communities to let their guard down. If individuals or communities believe they are protected but aren’t, the result can be more spread and more deaths.

In the fishing boat outbreak, two of the three people who tested positive for antibodies but did not have neutralizing antibodies got infected. There’s no way to know whether these patients’ results were false positives or weak positives, but the bottom line is clear: testing positive for antibodies does NOT mean you’re immune. Overconfidence can kill.

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The New York City health department is publishing great data on antibody testing. Overall, one fourth of tests are positive, although many undoubtedly are false positives. People in the neighborhoods and populations with the highest risk were least likely to get tested.

Throughout the US, those most at risk have the least access to care — this is known as the inverse care law and is a sad and accurate commentary on the injustice of the US healthcare system, to which New York City is no exception.

Crowding and herd immunity

Antibody-test positivity rates correlate closely with crowding and poverty. And although some communities may be approaching herd immunity, because communities mingle, New York City as a community is far from herd immunity.

Getting to herd immunity in the US would require hundreds of millions of infections and at least a million more deaths. That’s not a plan — that’s a catastrophe.

The promise of immunity, then, is that neutralizing antibodies protect against reinfection. Time will tell for how long protective immunity lasts and the proportion of people who need to be immune to drive down spread of infection substantially (herd immunity).

Vaccination will not be quick or easy, but it is likely to be feasible. Now we must develop a vaccine that is effective, safe, trusted, and widely available. If a safe and effective vaccine is widely available, immunity passports may become a reality.

We can get to a new normal if we improve control today, stay apart, work together, and prepare for vaccination if and when it becomes available. For now, we need to continue to wear masks, watch our distance, and wash our hands.

Our schools and universities need to be extraordinarily careful. In most of the country, we need to keep our bars closed, indoor dining closed, and avoid closed indoor spaces with lots of people present. There may be light at the end of the tunnel, but immunity is definitely not just around the corner!

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