Memory T cells and Covid-19: has Sweden reached herd immunity?

While “herd immunity” was mentioned as a viable strategy against Covid-19 early this year, only Sweden has chosen to develop immunity in this natural way and not go into full lockdown until a vaccine is available. Research on so-called memory T cells by Sweden’s Karolinska Institute now suggests that Sweden may indeed have reached herd immunity.

While the term “herd immunity” was frequently heard earlier this year as a viable strategy against corona, many countries considered this too dangerous and opted for lockdown and quarantine measures until a vaccine becomes available. Sweden was a notable exception by opting for a partial and voluntary lockdown. Sebastian Rushworth, a medical doctor in Stockholm, has explained what this kind of lockdown entailed. Based on his experience this year, he also makes the remarkable claim that Sweden has reached herd immunity – as have countries like England and Italy, without knowing it.

His claim appears to be confirmed by research on so-called T cells. An article in Nature Reviews Immunology stresses the importance of the contribution of B cells and T cells to immunity to SARS-CoV-2 and the implications for the development of effective treatments and vaccines for Covid-19. The authors emphasize the difference between antibodies and B and T cells by explaining that the creation of SARS-CoV-2-specific memory T cells and B cells (as apposed to circulating antibodies) is important for long-term protection. While reports have suggested that antibodies to the virus may only be maintained for a short period, causing speculation that immunity to the virus may also be short-lived, the memory B cells and T cells may be maintained even if there are no measurable levels of serum antibodies. Antibody responses may not be detectable anyway in patients with less severe forms of Covid-19.

The above is confirmed by Swedish Karolinska Covid-19 study group, whose research has shown that “SARS-CoV-2-specific memory T cells will likely prove critical for long-term immune protection against COVID-19”. The importance of T cells was already demonstrated in an article published this summer which notes that studies of people who were infected with SARS-CoV-1 in 2003 still have T-cells seventeen years after being infected. The Karolinska group appears to conclude similarly in respect of SARS-CoV-2 by observing that it “elicits robust memory T cell responses akin to those observed in the context of successful vaccines, suggesting that natural exposure or infection may prevent recurrent episodes of severe Covid-19”.

For the purpose of research on T cells, Sweden’s “open” strategy in relation to Covid-19 has been a blessing because it allowed for a more durable spread of the virus compared to other European countries. Also, individuals in the convalescent phase after showing asymptomatic/mild Covid-19 after returning to Sweden from endemic areas such as Northern Italy “exhibited robust memory T cell responses months after infection, even in the absence of detectable circulating antibodies specific for SARS-CoV-2, indicating a previously unanticipated degree of population-level immunity against COVID-19”. The paper’s conclusion, therefore, is that “individuals with asymptomatic or mild Covid-19 generated highly functional durable memory T cell responses …, [suggesting] that natural exposure or infection could prevent recurrent episodes of severe Covid-19”.

Returning to Rushford’s experience, he tells that Covid-19 is no longer front page news in Sweden and that he has not seen a single Covid patient in the Emergency Room in over two and a half months. Corona deaths in Sweden have dropped from over 100 a day at the peak in April, to around five per day in August. The few formal restrictions that applied under the partial lockdown are now gradually lifted.

Rushford thinks that countries that have experienced similar mortality curves as Sweden will also have likely developed herd immunity by now. However, countries that have successfully contained the spread of the disease, like Germany, Denmark and Australia, will have to stay in lockdown for at least another year, and possibly several years, if they don’t want to develop herd immunity the natural way.