Policemen and staff workers get tested for the COVID-19 disease at a makeshift nucleic acid testing centre in Shanghai, China March 24, 2022. Photo: Reuters/Aly Song
- Scientists use ‘hybrid immunity’ to mean the immunity acquired from natural infection and full vaccination.
- Two recent studies in The Lancet Infectious Diseases journal said hybrid immunity has proved good enough to reduce hospitalisation and deaths in COVID-19 cases.
- Whether hybrid immunity can preclude the need for booster doses isn’t yet known – and is the subject of ongoing study.
New Delhi: Two studies published last week in The Lancet Infectious Diseases said hybrid immunity was able to reduce the risk of severe disease and hospitalisation against COVID-19. The studies were conducted in Brazil and Sweden.
The term ‘hybrid immunity’ doesn’t exist in science textbooks. Scientists use it to mean the immunity acquired from two sources: natural infection and full vaccination. A section of scientists is now betting on hybrid immunity – also called “super-immune” immunity – to shape the future course of the COVID-19 pandemic, and our response to it.
On February 27, WHO chief scientist Soumya Swaminathan called hybrid immunity “the best kind of immunity to have”.
Once a healthy individual – who has neither had COVID-19 nor has been vaccinated – is exposed to SARS-CoV-2, her immune system produces antibodies that immediately fight the virus. Separately, the body also activates the memory T cells and B cells, which deal with cells that have been infected by the virus as well as develop a ‘memory’ of the infection in case the virus attacks again. This is broadly how natural immunity works.
On the other hand, a vaccine introduces an inactivated virus or a (dead) part of it into the body. This also prompts the immune system to trigger a response against the virus the way a natural infection does.
The protection granted by vaccines together with that due to a natural infection together make up hybrid immunity.
There has been one study on hybrid immunity in India. Padmanabha Shenoy, medical director of CARE Hospital, Cochin, conducted the study. He divided participants into four groups: those who had COVID-19 and have received one dose; those who had COVID-19 but haven’t received any doses; those who haven’t had COVID-19 and had received one dose; and those who haven’t had COVID-19 and had received both doses.
The results, published in The Lancet in November 2021, said that the bodies of people who had a prior infection and had received at least one vaccine dose had produced the highest level of antibodies.
“We found that if someone were to contract SARS-CoV-2 first, and then receive a single dose of the vaccine, this mounted a hybrid immune response that is 20-30 times stronger than [in] those who have been vaccinated, even with two doses” and without contracting the virus, Shenoy told the The BMJ in December.
The study also seemed to indicate that hybrid immunity could suffice to save us from severe disease and hospitalisation by COVID-19. This in turn precluded the need for booster doses.
However, the real challenges have to do with whether hybrid immunity is long-term and if it can protect sufficiently against newer variants of the novel coronavirus. Scientists are looking for the answer to the first question.
The second one is hard to study. When the omicron variant emerged, multiple studies concluded that it could weaken the body’s antibody response but that T- and B-cells were able to hold up. In addition, the omicron variant itself wasn’t capable of causing severe disease.
Our current crop of COVID-19 vaccines are designed to prevent severe disease and hospitalisation – and not prevent or improve other outcomes. If researchers start to design vaccines that can prevent infection as well, they will have to further study the hybrid immunity model. Then, we’re likely to have answers to all our questions – including about the need for booster doses.