In general, having antibodies for a specific virus is a good thing; they exist to the attack the virus, and are pretty good at it. But viruses are hackers, and some have evolved to do their best virusing when faced with an immune response.

Antibody-dependent enhancement (ADE) refers to any case in which virus-specific antibodies enhance the entry or replication of virus. Currently, we've found viruses that specifically target receptors on the surface of monocytes, macrophages and granulocytic cells, often by targeting the Fc receptors that the antibodies use bind to the virus.

Most viruses do not cause ADE, but neither is it exceptionally rare. One of the better-known diseases to exhibit ADE is the dengue virus. Dengue is one of the most common infectious diseases in the world, killing tens of thousands of people each year. In the case of dengue, we have identified four different serotypes; infection with any given serotype generally leads to only a mild infection, but reinfections with a second serotype leads to that virus attacking the body's antibodies, often leading to dengue hemorrhagic fever.

It occasionally happens that a vaccine accidently leads to ADE; in the 1960s specific vaccines for Respiratory syncytial virus (RSV) and Measles were found to lead to more severe infections if children were exposed months or years later. In 2016 a dengue vaccine drive proceed the death of 14 vaccinated children from dengue; this may or may not have been an ADE issue. Modern vaccines regularly check for ADE effects, and those that cause these effects do not make it past phase three trials.

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While there are some concerns that Covid-19 might possibly show ADE upon reinfection, we currently do not have enough clear examples of Covid-19 reinfection to have good data. In fact, there is some evidence that at least some cases of reported reinfection may actually be reactivation of the pre-existing infection, muddying the water even further. Likewise, we have no data to support any instances of ADE following vaccination for Covid-19, but we also do not have very good data on some of the vaccines out there; Russia's Sputnik V vaccine, in particular, faced criticism for rushing through trials. The Sinovac, Moderna, Pfizer, and AstraZeneca vaccines have been more fully tested, but are being continuously monitored for any signs of ADE responses.

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