Wait. I can catch Covid twice? my 50-year-old patient asked in disbelief. It was the beginning of July, and he had just tested positive for SARS-CoV-2, the virus that causes Covid-19, for a second time three months after a previous infection.

While theres still much we dont understand about immunity to this new illness, a small but growing number of cases like his suggest the answer is yes.

Covid-19 may also be much worse the second time around. During his first infection, my patient experienced a mild cough and sore throat. His second infection, in contrast, was marked by a high fever, shortness of breath, and hypoxia, resulting in multiple trips to the hospital.

Recent reports and conversations with physician colleagues suggest my patient is not alone. Two patients in New Jersey, for instance, appear to have contracted Covid-19 a second time almost two months after fully recovering from their first infection. Daniel Griffin, a physician and researcher at Columbia in New York, recently described a case of presumed reinfection on the This Week in Virology podcast.

It is possible, but unlikely, that my patient had a single infection that lasted three months. Some Covid-19 patients (now dubbed long haulers) do appear to suffer persistent infections and symptoms.

My patient, however, cleared his infection he had two negative PCR tests after his first infection and felt healthy for nearly six weeks.

I believe it is far more likely that my patient fully recovered from his first infection, then caught Covid-19 a second time after being exposed to a young adult family member with the virus. He was unable to get an antibody test after his first infection, so we do not know whether his immune system mounted an effective antibody response or not.

Regardless, the limited research so far on recovered Covid-19 patients shows that not all patients develop antibodies after infection. Some patients, and particularly those who never develop symptoms, mount an antibody response immediately after infection only to have it wane quickly afterward an issue of increasing scientific concern.

Whats more, repeat infections in a short time period are a feature of many viruses, including other coronaviruses. So if some Covid-19 patients are getting reinfected after a second exposure, it would not be particularly unusual.

In general, the unknowns of immune responses to SARS-CoV-2 currently outweigh the knowns. We do not know how much immunity to expect once someone is infected with the virus, we do not know how long that immunity may last, and we do not know how many antibodies are needed to mount an effective response. And although there is some hope regarding cellular immunity (including T-cell responses) in the absence of a durable antibody response, the early evidence of reinfections puts the effectiveness of these immune responses in question as well.

Also troubling is that my patients case, and others like his, may dim the hope for natural herd immunity. Herd immunity depends on the theory that our immune systems, once exposed to a pathogen, will collectively protect us as a community from reinfection and further spread.

There are several pathways out of this pandemic, including safe, effective, and available therapeutics and vaccines, as well as herd immunity (or some combination thereof).

Experts generally consider natural herd immunity a worst-case scenario back-up plan. It requires mass infection (and, in the case of Covid-19, massive loss of life because of the diseases fatality rate) before protection takes hold. Herd immunity was promoted by experts in Sweden and (early on in the pandemic) in the UK, with devastating results.

Still, the dream of herd immunity, and the protection that a Covid-19 infection, or a positive antibody test, promises to provide, have taken hold among the public. As the collective reasoning has gone, the silver lining of surviving a Covid-19 infection (without debilitating side effects) is twofold: Survivors will not get infected again, nor will they pose a threat of passing the virus to their communities, workplaces, and loved ones.

While recent studies and reports have already questioned our ability to achieve herd immunity, our national discourse retains an implicit hope that herd immunity is possible. In recent weeks, leading medical experts have implied that the current surge in cases might lead to herd immunity by early 2021, and a July 6 opinion piece in the Wall Street Journal was similarly optimistic.

This wishful thinking is harmful. It risks incentivizing bad behavior. The rare but concerning Covid parties, where people are gathering to deliberately get infected with the virus, and large gatherings without masks, are considered by some to be the fastest way out of the pandemic, personally and as a community. Rather than trying to wish ourselves out of scientific realities, we must acknowledge the mounting evidence that challenges these ideas.

In my opinion, my patients experience serves as a warning sign on several fronts.

First, the trajectory of a moderate initial infection followed by a severe reinfection suggests that this novel coronavirus might share some tendencies of other viruses such as dengue fever, where you can suffer more severe illness each time you contract the disease.

Second, despite scientific hopes for either antibody-mediated or cellular immunity, the severity of my patients second bout with Covid-19 suggests that such responses may not be as robust as we hope.

Third, many people may let their guard down after being infected, because they believe they are either immune or incapable of contributing to community spread. As my patients case demonstrates, these assumptions risk both their own health and the health of those near them.

Last, if reinfection is possible on such a short timeline, there are implications for the efficacy and durability of vaccines developed to fight the disease.

I am aware that my patient represents a sample size of one, but taken together with other emerging examples, outlier stories like his are a warning sign of a potential pattern. If my patient is not, in fact, an exception, but instead proves the rule, then many people could catch Covid-19 more than once, and with unpredictable severity.

With no certainty of personal immunity nor relief through herd immunity, the hard work of beating this pandemic together continues. Our efforts must go beyond simply waiting for effective treatments and vaccines. They must include continued prevention through the use of medically proven face masks, face shields, hand-washing, and physical distancing, as well as wide-scale testing, tracing, and isolation of new cases.

This is a novel disease: Learning curves are steep, and we must pay attention to the inconvenient truths as they arise. Natural herd immunity is almost certainly beyond our grasp. We cannot place our hopes on it.

D. Clay Ackerly, MD, MSc, is an internal medicine and primary care physician practicing in Washington, DC. He has served both as a faculty member of Harvard Medical School and as Assistant Chief Medical Officer at the Massachusetts General Hospital. He has also held positions in the government and private sector, including the White House, the Food and Drug Administration, and, most recently, as Chief Medical Officer of Privia Health. He can be reached at dclayackerly@gmail.com.

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Getting Covid-19 twice: Why I think my patient was reinfected - Vox.com