T cell immune responses seen a year after infection – The Hindu

Neutralising antibodies were detectable even 12 months after infection in most individuals.
Like in most countries where the Omicron variant had become dominant and caused a high spike in daily cases, the third wave in India propelled by Omicron caused a large number of reinfections in unvaccinated people and breakthrough infections even among the fully vaccinated. However, across the world, the Omicron variant was found to cause only mild disease in fully vaccinated people and in those with previous infection. This was real-world proof that previous infection and/or full vaccination with two doses provide protection against progression of disease to a severe form.
Laboratory studies undertaken in all countries have only studied the neutralisation ability of sera of people who have recovered from COVID-19 and people who have been fully vaccinated. This could only shed light on the ability of past infection and/or vaccination to prevent infection by highly transmissive variants with immune escape. But no studies have been done to evaluate the protective effect of memory T cell immune responses against severe disease 12 months after primary infection. A new study from Wuhan addresses this gap. The results were published in the journal The Lancet Microbe.
The researchers found that neutralising antibodies were detectable even 12 months after infection in “most individuals”, and it remained stable 6-12 months after initial infection in people younger than 60 years. The researchers found that “multifunctional T cell responses were detected for all SARS-CoV-2 viral proteins tested”.
And most importantly, the magnitude of T cell responses did not show any difference immaterial of how severe the disease was. While the ability of antibodies to neutralise was nearly absent against the Beta variant, it was reduced in the case of the Delta variant.
In contrast, the T cell immune responses were detectable in all the 141 individuals tested 12 months after infection and even when they had lost the neutralising antibody response. And the T cell responses were responding against the Beta variant in most of the 141 individuals.
“SARS-CoV-2-specific neutralising antibody and T cell responses were retained 12 months after initial infection. Neutralising antibodies to the D614G, Beta, and Delta were reduced compared with those for the original strain, and were diminished in general. Memory T cell responses to the original strain were not disrupted by new variants,” they write. “Our findings show that robust antibody and T cell immunity against SARS-CoV-2 is present in majority of recovered patients 12 months after moderate-to-critical infection.”
The study reveals the durability and robustness of the T cell responses against variants, including Delta, even after one year of infection. Most importantly, the robust and longstanding T cell responses were seen in people who have not been reinfected or vaccinated. This would mean even in the absence of vaccination, a person who has been infected by the virus even one year ago would have robust immune responses, which would offer protection against disease progressing to a severe form requiring hospitalisation. But the neutralising antibodies were found to diminish at the end of 12 months.
It might be recalled that except the Oxford vaccine (AstraZeneca), none of the trials evaluated the ability of the vaccines to prevent infection. The endpoint of all vaccine efficacy studies was to evaluate if vaccinated people developed symptomatic disease or not.
However, the booster doses aggressively pushed by vaccine manufacturers are for preventing infection. And even when the neutralising antibodies increase after a booster shot, they do drop after a few months. No studies have been done to evaluate if booster doses improve T cell immune responses, which is the most important criterion of vaccination.
In the case of neutralising antibodies, the researchers found that 121 (85.8%) were positive for neutralising antibodies at the 6 months while there was a slight reduction at the end of 12 months as only 115 (81.6%) were positive for neutralising antibodies.
The neutralising antibody titres did not show any difference based on disease severity — mild or moderate — or in those younger than 60 years. However, the neutralising antibody titres declined in older people and in people with critical disease.
A year after infection, 115 of 141 (82%) individuals had neutralising antibodies against the original strain from Wuhan, China. “In contrast, only 68 (48%) had neutralising antibodies against D614G, 32 (23%) had neutralising antibodies against the Beta variant, and 69 (49%) had neutralising antibody responses against the Delta variant”, they write.

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Printable version | Mar 26, 2022 7:20:25 pm | https://www.thehindu.com/sci-tech/t-cell-immune-responses-seen-a-year-after-infection/article65259153.ece
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