• Some people may be immune to COVID-19 for an unexpected reason.
  • A new coronavirus immunity study delivers the same conclusion similar papers have offered in the past few months. They suggest that the immune system might get an unexpected boost from the common cold.
  • Researchers looking to answer questions surrounding COVID-19 immunity discovered that immune responses to other human coronavirus infections generated T cells that are capable of recognizing the COVID-19 virus and mounting a rapid response.
  • These T cells can roam the body for years and provide a prolonged defense against the novel coronavirus. It’s still unknown how long COVID-19 immunity might last, and that’s a crucial detail for future pandemic and vaccination management policies.

Gaining immunity to the novel coronavirus is the ultimate goal for public health officials. This can happen in two ways, via direct infection or immunization. The latter is the preferred means, as extensive vaccination campaigns can deliver the herd immunity phenomenon that will stop the virus from spreading so quickly. With fewer susceptible targets out there, the virus would have a hard time jumping to new hosts. COVID-19 might not disappear entirely, but therapeutics that prevent complications and deaths — and vaccines that might prevent the infection entirely — could make the disease much less dangerous. Before any of that can happen, scientists need to answer a crucial question: how long does COVID-19 immunity last? Regardless of how it’s acquired, we need to know how long people are protected against reinfection so that appropriate strategies can be formulated.

Some of the scientists who are looking to explain and quantify COVID-19 immunity have reached the same interesting conclusion. There may be a third way to become immune to COVID-19 aside from surviving the disease or receiving a vaccine. Previous exposure to the common cold may train the immune system to recognize and neutralize SARS-CoV-2 as well.

Four known human coronaviruses cause the common cold, and a study said a few days ago infection from any of these viruses might teach the immune system to recognize the novel coronavirus and prevent it from causing complications. Researchers from the Duke-NUS Medical School explained that T cells created by the immune system to beat the common cold can boost the immune response against COVID-19. The team found that people who were infected with SARS in 2003 still had circulating T cells some 17 years later.

The T cells are white blood cells that are roaming the body, always in search of a specific pathogen they’ve been trained to remember. Upon secondary contact, they can create more T cells that would then neutralize infected cells. T cells could also recruit B cells that are responsible for the creation of new antibodies.

Researches a few weeks ago said that COVID-19 antibodies might disappear from the bloodstream just three months after an infection. But we learned at the time that the immune system also trains T cells that would linger on. These cells are not detected via antibody tests meant to confirm whether a person survived COVID-19. But they exist and can be identified with more complex tests. Some of the promising vaccine candidates deliver the same dual defense mechanism, raising antibodies that can block the virus as well as T cells that can remember the encounter.

The newest study on the matter was published in Science, and it comes from the La Jolla Institute for Immunology researchers who proved the same phenomenon back in May. Around the same time, a study from Germany’s Charité University Hospital indicated the importance of T cells in COVID-19 immunity.

The La Jolla scientists think that people who have never been exposed to COVID-19 may deliver a better immune response to the virus because of their previous colds. Those episodes trained a generation of T cells that can also identify SARS-CoV-2 and mount a fast response. “This could help explain why some people show milder symptoms of the disease while others get severely sick,” coauthor of the study Alessandro Sette said in a press release. However, it’s unclear how preexisting immunity affects the COVID-19 prognosis.

Sette and his team looked at blood samples collected between 2015 and 2018 from 25 people, at a time when the COVID-19 virus wasn’t circulating in humans. The researchers discovered that T cells in those samples could recognize the new coronavirus as well as the four types of known human coronaviruses that cause the common cold.

While that’s great news for managing COVID-19 patients, more research is needed to determine exactly what sort of protection previous exposure to known coronaviruses offers against COVID-19. More research could also explain whether there’s any correlation between exposure to one of four mild coronaviruses and the evolution of the COVID-19 infection. Additionally, it would be interesting to see whether this cross-reactive T cell response can explain why some people do not develop symptoms and recover faster than others.