Why are boosters needed

Among those who do have a strong response to a two-dose vaccine, their level of protection against infection or serious disease is being tracked over time. Earlier this year, Israel reported increasing rates of infection in fully vaccinated people aged 60 years and older. This led the government to provide third doses for this age group. In the short-term, the strategy appears to have worked, with infections dropping ten-fold at least two weeks after the boost.

Across a population, we can track how well vaccines are performing at preventing people from getting infected, getting sick, or needing to go to the hospital. There is evidence of gradual decreasing vaccine effectiveness over time. However, the ability of vaccination to prevent hospitalisation from COVID remains very high even after six months. There are two key parts to this immunity: the antibodies that can bind to the virus and stop infection completely, and the cells that remember the virus for hopefully years to come, ready to be reactivated if the virus gets in.

After a few months, the levels of these antibodies have dropped somewhat among those who receive two doses, likely explaining why vaccine effectiveness declines and breakthrough infections in fully vaccinated people occur.

But if our immunity drops, why are people still protected from hospitalisation and severe disease? If you do get infected after being vaccinated, your white blood cells will quickly jump into action, producing lots of antibodies and getting ready to kill the virus.

Although longer-term immunity from vaccination dramatically reduces the need for hospitalisation, breakthrough infections following the waning of immunity do result in further spread of the virus , complicating efforts to control the epidemic. Therefore, administering boosters will likely reduce infection and transmission, but the effect of boosters to prevent serious disease and death is more modest, at least in those under Shaw says.

Doctors use the term third dose when referring to people with compromised immune systems who may not have gotten the level of protection they need from the first two doses. The third dose provides that level of immunity. A booster shot is recommended due to concern that the effectiveness of the vaccine decreases over time and may not protect against a new strain, such as Delta.

A booster may be given to older people or those with chronic medical conditions or other risk factors. While a booster sometimes is an exact replica of the initial vaccine, it can also be tweaked. With COVID, this is key because the vaccine could then be tailored to target particular variants of the virus. Shaw adds. It is normal for virus-fighting antibodies—such as those that are stimulated by a COVID vaccine—to wane over time.

Monitoring antibody levels in the blood is one way to measure vaccine efficacy and research has found that protection remains high for six months after the second shot of a Pfizer or Moderna vaccine. If you are re-exposed to something [via a booster shot, that follows the original exposure by vaccination], the memory response is even more vigorous than the original.

This memory response includes antibody responses, but also includes an additional arm of the immune system controlled by a different group of white blood cells called T cells or T lymphocytes. T cell immune responses are especially important for viral infections like those with SARS-CoV-2, but are more difficult to study than antibody responses outside of a research laboratory setting, adds Dr. This improves or gives the immune system a boost. Centers for Disease Control and Prevention that moderately to severely immunocompromised individuals who received the two-dose series of either Pfizer or Moderna get a third dose of the vaccine 28 days after receiving their second dose.

These individuals are especially vulnerable to COVID and are more at risk of serious, prolonged illness.