How immunity is developed?

A longitudinal study by the researchers from King’s College London has suggested that immunity to Covid-19 might be lost in months. The suggestion is based on a steep drop in patients’ antibody levels three months after the infection.

• The researchers analysed the immune response of 90 recovered Covid-19 patients and healthcare workers. In their study, they found that antibodies that specifically help in neutralising the coronavirus SARS-CoV-2 decreased 2-23-fold during an 18-65 day follow-up period. This is similar to the decrease in antibodies seen in seasonal coronaviruses associated with the common cold.
• The researchers suggested that immunity developed by the formation of antibodies against SARS-CoV-2 lasts only a few months, and recovered Covid-19 patients are likely to remain susceptible and could get re-infected.
• The analysis found “a potent” level of antibodies produced in 60% of participants during the peak of their infection; and that only 16.7% retained that level of potency 65 days later.
  • While the level of antibodies was at a higher level in patients with severe symptoms, the researchers said it is not clear why antibody response correlates with disease severity.
  • The researchers noted that asymptomatic individuals also generate antibodies against the virus.

Does a decrease in antibodies result in losing immunity against the virus?

• Not necessarily. Antibodies are like fingerprints that give us evidence that a pathogen has caused an infection and that the immune system has responded.
• The antibodies usually remain in the blood for some time and quickly activate the immune system when the body is exposed to the pathogen again.
  • Some antibodies not only recognise when the pathogen returns, but also protect the body for a lifetime from re-infection, as in the case of measles.
  • However, in the case of seasonal flu, the antibodies give protection for a very small period.
  • In the case of the novel coronavirus, it is not yet clear how long the antibodies provide recovered person protection.
• Further studies are needed to determine the longevity of the antibody response and what level of antibodies is required to protect against re-infection.
• The researchers flagged the fact that antibodies are just one of the ways in which the body can fight a virus such as SARS-CoV-2.

Other Methods

• T cells are a key component of the immune response against a virus. A human body has two mechanisms of immunity response within the adaptive immune system.
  • First is the humeral immune, which is also called antibody-mediated immunity (as discussed above).
  • When a virus enters a cell and can no longer be detected by antibody-mediated immunity, a cell-mediated immune response can take over to kill the virus.
• Cellular immunity occurs inside the infected cell and is mediated by cells called T lymphocytes. These are the T cells, which recognise the infected cell. Before that, these cells have to be activated by interacting with an antigen (virus)-presenting cell. Once the T cells are activated, they clone themselves generating numerous T cells, and destroy the infected cells.
• During the process of activation, some of the T cells remain inactive as memory cells. These produce more T cells if the infection returns. Therefore, memory plays a crucial role in providing cell-mediated immunity.
  • There are also helper T cells, which function indirectly by communicating with other immune cells about potential pathogens.

Conclusion

• More research is required to answer questions like what is the contribution of T cells to initial virus control and tissue damage in the context of Covid-19; and how do memory T cells established thereafter contribute to protective immunity upon reinfection.
• While the induction of robust T cell immunity is likely essential for efficient virus control, dysregulated T cell responses may cause immunopathology and contribute to disease severity in Covid-19.