new study in Sweden is raising hopes immunity to Covid-19 among the public is “probably significantly higher” than has been suggested by the results of testing for antibodies of the disease.
Sweden has taken a less stringent approach than many other countries to restricting the spread of the Sars-CoV-2 coronavirus that causes Covid-19.
The Swedish approach has widely been described as herd immunity, although the country’s health officials don’t describe it that way.
Critics of the approach have pointed to research showing fewer Swedes than expected have developed Covid-19 antibodies as part of the evidence the approach isn’t working.
Results presented in mid-June, based on more than 4000 blood samples taken across the country between late-April and late-May, put the proportion of the Swedish population with the antibodies at 6.1 per cent.
The new research, from the Karolinska Institutet and Karolinska University Hospital, suggests the antibody data does not tell the full story.
The researchers analysed samples from 203 people and said their results indicated roughly twice as many people had developed Covid-19 immunity through T-cells, than had developed detectable antibodies.
T-cells are a type of white blood cell specialised in recognising virus-infected cells, and are an essential part of the immune system. When they encounter antigens – the molecules that fight off viruses – they are programmed to fight the same or similar viruses.
About 30 per cent of donors in the study who had given blood in May 2020 had Covid-19-specific T-Cells, the Swedish study found.
Patients with severe Covid-19 often developed a strong T-cell response and an antibody response, a press release on the research said.
In those with milder symptoms it was not always possible to detect an antibody response, but many still showed a marked T-cell response.
T-cell immunity was also found in people who had no Covid-19 symptoms but who had been exposed to family members known to be infected.
T-cell analyses were more complicated to perform than antibody tests and only done in specialised laboratories, the release said.
“Our results indicate that public immunity to Covid-19 is probably significantly higher than antibody tests have suggested,” Professor Hans-Gustaf Ljunggren, from the Centre for Infectious Medicine at the Karolinska Institutet, and a co-senior author of the study, said.
”If this is the case, it is of course very good news from a public health perspective.”
The article said that without a vaccine for Covid-19, it was critical to determine whether exposed and infected people – specially those without symptoms or only very mild forms of the disease – developed robust immunity.
The T-cell response to Sars-CoV-2 was similar to the reaction seen to successful vaccines for other infections, the report said.
That suggested natural exposure or infection may prevent further severe Covid-19 infections in people who had not been found to develop antibodies.
Another issue to consider was that after infection with the Sars-CoV-1 virus, the response from memory B cells –that produce antibodies – tended to be short-lived after infection. In contrast, memory T-cell responses could persist for many years.
The article said it remained to be determined whether a robust T-cell response to Sars-CoV-2, without detectable antibodies, could protect against the disease.
But it also noted none of the people in the study who had recovered from Covid-19, including those who had mild cases or were asymptomatic, had caught the disease again.