COVID-19 Reinfection: T Cell Immunity & the COVID-19 Vaccine

March 2021

COVID-19 Cells

COVID-19 Reinfection

In January 2020, the Center for Disease Control (CDC) confirmed the first domestic case of COVID-19, an infectious disease caused by the SARS-CoV-2 virus. Soon after, COVID-19 was declared a global pandemic, spreading at exponential rates and drastically altering the world as we knew it. After more than a year of research and experiments, we’ve begun to learn more about the mechanisms of this coronavirus variant, and how our bodies fight it.

Unfortunately, the disease hasn’t yet been around long enough for us to have a complete grasp on reinfection. Reinfection occurs when an individual who’s already gotten the disease is infected again after a period of immunity. The timeline for a patient being reinfected holds massive implications about how the U.S. should carry out its response to the virus.


According to the CDC, as of January 2021 there have been almost 27 million reported cases of COVID-19 in the U.S. A majority of cases are mild to moderate, involving a fever, cough, and fatigue that typically lasts between 2 to 4 weeks. Some more severe cases, however, can aggravate other conditions or cause more complications and plague the body for months. Recovery time depends heavily on age, strand, and other health factors.

Regardless of the duration and severity, after battling the SARS-CoV-2 virus a patient will develop an immune response to deal with future exposure called a natural immunity. 

Natural Immunity

Even after an individual recovers, their immune system is prepared to fight the specific pathogens that attacked it for a period of time afterwards. Many studies have been done on COVID-19 to try to figure out exactly how long this natural immunity lasts, but due to individual differences and strain variations, it’s hard to determine an absolute value. For example:

  • One study published October 12, 2020 showed that elevated levels of COVID-19 specific antibodies were present in the body for up to 3 months after the initial infection. However, this study did not persist long enough to test the full capacity, stopping after roughly 100 days.
  • Another study published January 15, 2021 that followed UK healthcare workers concluded that individuals who have been infected have an 83% reduced risk of getting the virus again for at least 5 months. While reinfections are possible, they are extremely rare and are reported to result in much milder forms of COVID-19.
  • Published even more recently, a study released February 5, 2021 studied the immune memory of specific COVID-19 antigens in the immune system. This study found that roughly 95% of subjects retained some level of immune memory until the end of the study at approximately 6 months.

As the global scientific community continues to learn more about lasting immunity, there remain many unanswered questions that are still under active investigation. The emergence of new strains in the UK, South Africa, and elsewhere also will play an important role in this ongoing research, as recovery from one strain of the disease may have protective effects against another variant, but possibly to a lesser degree. The disease itself is still relatively new, and we simply haven’t known about it long enough to have definitive answers about immunity yet – although we are getting closer every day.

Immune Involvement

Immune memory is a term used to describe the length of time certain cells in the body will be able to remember and effectively combat a particular pathogen. There are four main types of cells involved in immune memory, CD8+ T cells, CD4+ T cells, memory B cells, and antigen-specific antibodies. 

T Cells

T cells (link to T cell product category?) are lymphocytes that mature in the thymus and play a critical role in the immune response. Without T cells, humans would not be able to recover from sickness. There are multiple types of T cells that have different functions; when it comes to building immunities it’s important to focus on memory T cells. Memory T cells last after a disease is eradicated and are indicative of a successful long-term immune response.

When a harmful pathogen enters the human body, the immune system sends a variety of cells to locate, identify, and destroy it. This process can take a while depending on how complex or concentrated the pathogen is. Once the task is completed, most of the T cells are cleaned up by waste management in the bloodstream, but some stick around in case that same pathogen returns. Memory T cells possess the knowledge to recognize specific pathogens and the ability to transform instantly upon exposure. A memory T cell can differentiate into a CD4+ helper T cell (link to Human CD4+ isolation kit?) that helps alert the body of the infection, or a CD8+ cytotoxic T cell that attacks the infected cells. Building up a sufficient crop of memory T cells allows an individual to maintain immunity to a specific disease without having their immune system constantly activated.

B Cells and Antibodies

Similar to memory T cells, memory B cells remember specific pathogens to spring into action upon repeated exposure. B cells (link to b cell product category?) are responsible for producing antibodies to help identify and fight infection. When a virus returns to the body, a related memory B cell will be able to begin producing antigen-specific antibodies right away that can bind to the cells and stop them from spreading. The production of these antibodies are the main line of defense against reinfection.

COVID-19 Vaccines

Toward the end of 2020, the Food and Drug Administration (FDA) granted emergency use authorizations (EUAs) to two companies in the U.S. that developed vaccinations to help combat the COVID-19 pandemic. These were the first mRNA vaccines to ever be approved in the U.S. for commercial use and distribution. An mRNA vaccine is different from a traditional vaccine because it involves the injection of genetic material that will be turned into proteins, as opposed to injecting weakened versions of the pathogen.

The vaccines, engineered by Pfizer/BioNTech and Moderna, rely on the immune system to build an immunity.


Inside our cells, messenger ribonucleic acid (mRNA) is responsible for transporting genetic information from the nucleus to the ribosomes in the cytoplasm that are responsible for producing proteins. When used in a vaccine, strands of mRNA are sent into cells in capsules which cannot pass the nuclear membrane. This means the mRNA vaccine cannot affect our DNA.

Upon entering the cell, the genetic information is read by ribosomes to produce a spike protein specific to the SARS-CoV-2 virus. The virus typically uses this protein to latch onto healthy cells, but with no virus present, they are harmless. The immune system still recognizes these proteins as a foreign substance and develops antibodies to eradicate them from the body. While the T and B cells work together to eliminate all traces of the protein, the mRNA in the cell is cleaned up by lysosomes, leaving no trace of its presence.

The second dose of the vaccine, required a few weeks after the initial injection (depending on the brand), activates the memory cells and bolsters the immune response further, solidifying the robust production of COVID-19 specific antibodies. Though the vaccines are too new to have significant data on reinfection yet, early clinical trials indicate they will develop a longer lasting immunity than natural exposure to the virus.

Herd Immunity

The rapid spread of COVID-19 in combination with the fact that SARS-CoV-2 is an mRNA virus has led to a large number of mutations from the original strain. So far, the vaccines that have been developed seem to be providing a significant level of protective immunity against the newer known strains because they share the same spike protein. However, some of the newer strains are more contagious and are spreading at faster rates than the vaccines can be administered. As new strains become characterized, vaccine manufacturers can further refine the exact formulation of the vaccine to deliver the strongest immunity against dominant strains to best protect against infection.

Ultimately the goal is to reach herd immunity, which occurs when a large enough percentage of people in a population are immune to a disease to make infection unlikely. This protects an entire community even though not everybody is immune.

The mutation of strains beyond the scope of current vaccinations could significantly change the amount of time it takes to reach herd immunity. Herd immunity is also slowed by the potential for immune individuals to potentially spread the virus even though they do not contract the disease. Although early research indicates that it’s very rare for vaccinated individuals to have high enough concentrations of the SARS-CoV-2 virus to infect others, these studies are still ongoing and it’s still important to take other precautions like wearing a mask and social distancing, even after being vaccinated.

Akadeum Life Sciences

Even with everything we know about COVID-19, further research is required to better understand the  disease, how it spreads, and the immune response it stimulates. Studying T cells and B cells can give us an in-depth understanding of how the immune system reacts when exposed to the SARS-CoV-2 virus. To make research more efficient, Akadeum Life Sciences has developed a cell separation method that isolates immune cells quickly without damaging cell health or physiology.

By using microbubbles to bind to unwanted substances and float them to the top of the sample, Akadeum’s T and B cell isolation kits can separate target cells from residual blood cells in approximately 30 minutes without the need for additional equipment. Buoyancy-activated cell separation (BACS) works gently and directly in the sample container to maximize cell throughput for downstream applications.

If you’re looking to perform research on immune cells or further purify samples for downstream applications, explore our portfolio of immunology kits or contact a member of our scientific team to discuss your research application today.

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