Francisco R. Velázquez, M.D.: What happens to vaccine-induced immunity over time?

One of the most common questions we’re asked is, “How long does the immune response from a vaccine last?”

The answer is somewhat complicated, as it varies. It depends on the vaccine and the organism for which the vaccine was developed. To fully address this question, it is important to understand in general terms the immune response, how vaccines work and the different types of vaccines.

Infections are one of the most common causes of human disease. Lower respiratory tract infections are the fourth-leading cause of death in the world and diarrheal disease the eighth.

Organisms such as bacteria and viruses must overcome all the natural barriers to ultimately gain access to the tissue or cells in which the damage will result in disease. All viruses and a few bacteria must enter the cell to survive and use the cell’s own resources to replicate. The SARS-CoV-2 virus that causes COVID-19 is a good example.

Immunity can be short-lived or long-lasting depending on the antigen, amount of antigen and route used by the organism to gain access. As an example, the Human papillomavirus (HPV) vaccine induces an antibody response that lasts for decades, whereas the flu shot needs to be annually reconfigured because of the variability in seasonal strains.

An antigen is any substance that elicits an immune response, and a vaccine is a biological product that safely induces an immune response. Depending on the vaccine, a component, product, or biological marker is presented to the immune system which recognizes it as foreign and mounts an immune response. As a result, the protective antibodies and other specialized cells develop a long-term memory of the offending pathogen.

Which brings us to the question, “How long does the response last?” Within a couple of weeks after an immunization, the body releases a group of cells called B cells that generate the protective substance we call antibodies. These are specialized Y-shaped proteins, also known as immunoglobulins, that bind to a specific target on the foreign object. The two arms of the Y portion lock onto a specific antigen, such as the spike antigen, like a lock and key. The stem of the Y serves as a marker for other cells and molecules from the immune system. In addition to the B cells, the body generates another type of white blood cell called a T cell. In general, there are three types of these cells – named cytotoxic, regulatory and helper. As their names imply, each one has a specific function in the destruction of the infected cells. T cells are more specific as these are not activated until they find their specific antigen.

Vaccines in general offer both short- and long-term protection against infection, what we refer to as immunity. The initial response will decrease over time, which is normal and expected with all vaccines. These cells don’t reduce completely, and we are left with longer-lasting memory B and T cells, which remain ready to mount an immune response if the disease is encountered again.

That is the typical way vaccines work. Over time, the initial levels of antibodies decline. As an example, a recent study found that antibodies start to wane about 12 weeks after the second dose of vaccine. Similar studies have determined that antibodies will start to decrease by about 2% at peak level to about 25% at around six months after the vaccine is administered. Other studies have demonstrated that the number of memory B cells increases three to six months after the second dose of the m-RNA vaccines. The researchers also found a significant level of the helper and killer T cells. Combined, these findings point to an immune system that has been trained to respond if presented with the same agent again. It is important to remember vaccines typically protect you against severe disease and potential mortality, but no vaccine is 100% effective in preventing the possibility of infection.

The current COVID-19 vaccines have demonstrated an effective level of protection against severe disease-causing hospitalizations and mortality. We have seen that clearly in the recent surge caused by the delta variant. But like most vaccines produced against rapidly changing viruses, the need for either additional doses or boosters has been identified. So far, the COVID-19 vaccines are working as they should and are waning as expected. We still have much to learn about the waning timeline, but that shouldn’t discourage you from taking advantage of the protection it can offer you – and those around you – now.

Francisco R. Velázquez, M.D., S.M., is the Spokane Regional Health District health officer.