COVID 19: Can you get it twice?

There have been some case reports of individuals who have developed symptomatic COVID-19, tested positive, then negative, then several weeks later positive again. Does this mean they were re-infected? Probably not (but we are not certain).

For a great, short interview on science Friday that addresses this topic listen here.

This is an interview with Columbia University Virologist Angela Rasmussen.

Quick summary:

Some studies have demonstrated that “neutralizing antibodies” initially detected in recovered COVID 19 patients can disappear (not measurable) after several weeks. But this does not mean those patients do not have immunity.

The immune response involves several cell types including B memory cells, T memory cells, and T helper cells. When a previously infected patient no longer has detectable antibodies they can still have memory cells of both types as well as T helper cells. When challenged (exposed) to the virus again those memory cells can become activated. They can then proliferate and respond to the virus. The B memory cells produce antibodies and the T memory cells become “killer cells” which can kill infected cells, stopping replication of the virus and controlling infection.

A T helper cell is a type of immune cell that stimulates killer T cells, macrophages, and B cells to make immune responses. A helper T cell is a type of white blood cell and a type of lymphocyte. Also called CD4-positive T lymphocyte.

A study in South Korea looked at 300 patients who were infected, cleared the virus (symptoms resolved and PCR test turned negative) but then subsequently tested positive again (nasal PCR). All of the contacts for these patients were traced and there was no evidence of virus transmission from any of the 300 patients, suggesting that the positive PCR nasal swab represented non-infectious residual virus remnant particles. In addition, cell culture tests in all of these patients was negative for infectious virus, further pointing to false positive repeat tests in all 300 patients.

One study found that some patients with no symptoms of Covid-19 had T-cells that recognized the virus — even when they had no detectable antibodies. 


SARS-CoV-2-specific memory T cells will likely prove critical for long-term immune protection against COVID-19. We systematically mapped the functional and phenotypic landscape of SARS-CoV-2-specific T cell responses in a large cohort of unexposed individuals as well as exposed family members and individuals with acute or convalescent COVID-19. Acute phase SARS-CoV-2-specific T cells displayed a highly activated cytotoxic phenotype that correlated with various clinical markers of disease severity, whereas convalescent phase SARS-CoV-2-specific T cells were polyfunctional and displayed a stem-like memory phenotype. Importantly, SARS-CoV-2-specific T cells were detectable in antibody-seronegative family members and individuals with a history of asymptomatic or mild COVID-19. Our collective dataset shows that SARS-CoV-2 elicits robust memory T cell responses akin to those observed in the context of successful vaccines, suggesting that natural exposure or infection may prevent recurrent episodes of severe COVID-19 also in seronegative individuals.

The last sentence is good news, supporting immunity after infection or exposure with COVID-19 specific memory T cells in the absence of measurable antibodies. Future studies will tell us more about whether the presence of these T cells can prevent re-infection or at least limit the degree of illness.

There is also the possibility that there is some cross reactivity between one of the four common cold Corona viruses and the COVID 19 virus, as suggested by the low incidence of COVID 19 infection on an Island off the coast of Tuscany despite infected travelers from the mainland bringing the virus across to the island. The previous year the island had experienced a particularly bad bout of the common cold.

Another study of COVID-19 specific T cells in Sweden has recently supported sustained memory T cells in previously infected patients and their contacts in the absence of antibodies . It also demonstrated antibodies against COVID 19 in unexposed individuals suggesting cross reactive antibodies from previous infection with other corona viruses.

Targets of T Cell Responses to SARS-CoV-2 Coronavirus in Humans with COVID-19 Disease and Unexposed Individuals

Using HLA class I and II predicted peptide “megapools,” circulating SARS-CoV-2-specific CD8+ and CD4+ T cells were identified in ∼70% and 100% of COVID-19 convalescent patients, respectively. CD4+ T cell responses to spike, the main target of most vaccine efforts, were robust and correlated with the magnitude of the anti-SARS-CoV-2 IgG and IgA titers. The M, spike, and N proteins each accounted for 11%–27% of the total CD4+ response, with additional responses commonly targeting nsp3, nsp4, ORF3a, and ORF8, among others. For CD8+ T cells, spike and M were recognized, with at least eight SARS-CoV-2 ORFs targeted. Importantly, we detected SARS-CoV-2-reactive CD4+ T cells in ∼40%–60% of unexposed individuals, suggesting cross-reactive T cell recognition between circulating “common cold” coronaviruses and SARS-CoV-2.”

There is much more to learn about this novel virus. Because of it’s high lethal rate (25 times greater than the previous H1N1 flu virus pandemic) and it’s greater transmission capability (from asymptomatic as well as symptomatic patients, both droplet and aerosol transmission) intensive and unprecedented research efforts are being made.

There are 100 different vaccines under early study. Two vaccines (Oxford University and Moderna) have completed phase 1/2 and phase 1 studies respectively, demonstrating safety and anti-body responses. Phase 3 studies will determine whether and how effective they might be and provide more data on safety.

In the context of the COVID 19 pandemic I will close with the usual summary.

  1. Avoid alcohol consumption (alcohol wreaks havoc with your immunity)
  2. Get plenty of sleep (without adequate sleep your immune system does not work well )
  3. Follow good sleep habits
  4. Exercise, especially out of doors in a green space, supports the immune system
  5. Get some sunshine and make sure you have adequate Vitamin D levels.
  6. Eat an anti-inflammatory diet rich in micronutrients.
  7. Practice stress reduction like meditation and yoga which improves the immune system
  8. Eliminate sugar-added foods and beverages from your diet. These increase inflammation, cause metabolic dysfunction, and suppress immunity.
  9. Eliminate refined-inflammatory “vegetable oils” from your diet, instead eat healthy fat.
  10. Clean up your home environment and minimize your family’s exposure to environmental toxins by following recommendations at with regards to household products, personal care products, and organic foods. (


Eat clean, drink filtered water, love, laugh, exercise outdoors in a greenspace, get some morning sunlight, block the blue light before bed, engage in meaningful work, find a sense of purpose, spend time with those you love, AND sleep well tonight.

Doctor Bob

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