Amid the coronavirus disease (COVID-19) pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), it is still unclear how long an immune response against SARS-CoV-2 lasts. Recent studies have shown that the immune response may last up to about four months, which could help in the development of potential vaccines against the virus.
Now, an international team of thirty researchers from Sweden, Italy, Germany, the USA, and China aimed to determine the longevity of the immune response against SARS-CoV-2, particularly the specific memory B-cell and T-cell responses over time in convalescent COVID-19 patients.
The team has found that the levels of specific Immunoglobulin M (IgM) and Immunoglobulin A (IgA) antibodies declined after one month. In contrast, specific Immunoglobulin G (IgG) levels remained stable for up to six months after infection. Further, the team detected anti-SARS-CoV-2 IgG antibodies in serum samples collected six to eight months after the onset of symptoms.
The SARS-CoV-2 emerged in Wuhan City, China, in late December 2019. Since then, many developments have been made in finding out more about the virus, how it spreads, and if it induces a long-lasting immune response. However, it remains unclear whether the antibody titer is a marker for protective immunity against COVID-19.
A robust adaptive immune response and spike-specific neutralizing antibodies, circulating follicular helper T cells, and memory B cells have been detected in patients who recovered from COVID-19. It remains a mystery how long the adaptive immunity lasts after the natural infection.
To arrive at the study findings, which appeared on the pre-print server bioRxiv*, the researchers tested a cluster of patients from the Lombardy region, the hardest-hit region in the first wave pandemic in Italy. These patients experienced mild to critical illness due to COVID-19. Along with them are Swedish volunteers with mild symptoms.
The team detected the presence of elevated anti-spike (anti-S) and anti-receptor binding domain (anti-RBD) antibody levels over eight months. Also, specific memory B cell and T cell responses were evaluated in some patients.
What the team found
The research revealed that an anti-SARS-CoV-2 antibody response exists in most COVID-19 patients as early as two weeks after the start of their symptoms. Further, the level of anti-S and anti-RBD IgG remained stable for about six months after being diagnosed, followed by declining levels between six and eight months. However, the decline in anti-S and anti-RBD IgA and IgM levels were seen from one to three months after disease onset.
“Our results are in line with previous studies showing a similar longevity and pattern of anti-SARS-CoV-2 antibody response with antibody levels reaching a peak at 23 days following symptom onset and being maintained for at least 4 months,” the researchers explained.
The team also observed that men had higher anti-RBD IgG antibody titers, especially those who were more severely ill with COVID-19. Meanwhile, 7 percent of patients who developed severe illness did not develop or had an extremely low level of antibodies after being infected. This means that these patients’ immune systems exhibited a weaker response.
“The presence of high level of SARS-CoV-2 specific memory B and T cells in the majority of patients, 6- 8 months after infection, suggests that immunity after infection could be at least transiently protective and that development of long-term protective immunity through vaccination might be possible,” the researchers concluded.
Determining the longevity of the body’s immune response is crucial in the development of effective vaccines. With a longer immunity against SARS-CoV-2, the vaccines being developed will be more effective in inducing an immune response.
Currently, there are over 200 candidate vaccines against SARS-CoV-2. Of these, 47 are undergoing trials to test the vaccines’ efficacy and safety. Ten vaccines are in the last phase of human trials.
To date, there are have been over 50.8 million confirmed infections with SARS-CoV-2 worldwide, and at least 1.26 million deaths. More than 33.23 million have already recovered. However, a second wave of the outbreak is happening in many countries across the globe, which is likely to raise the numbers considerably in the coming weeks.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.