Naturally acquired immunity protects from severe COVID upon reinfection: statistical evidence from Quatar

A recent study by Abu-Raddad et al., published in the formerly respectable New England Journal of Medicine [1], shows that acquired immunity against COVID works very much as it does with other respiratory viruses: it affords relative protection against reinfection and robust protection against severe disease or death.

The study was conducted on a large cohort of approximately 260,000 unvaccinated persons,1 and it used sensible cut-off values for the diagnostic PCR test. It also has a careful statistical design to correct for various compounding factors, including comorbidity and changes in the mix of viral variants between the first and the second infection. Key findings:

  • Reinfections were rare—there were 1,304 cases overall, which amounts to approximately 0.5% of all individuals included in the study.
  • Out of 1,304 patients with reinfection, only 4 were admitted to the hospital; none of them required ICU treatment, and none died either.
  • In the control group of patients with primary infections, 158 out of 6095 individuals required hospital admission; 28 of these received intensive care, and 7 died.

The clear difference in disease severity between the first and the second infection suggests that at least the first PCR diagnosis was true-positive (and not false-positive) in most cases; otherwise, we should expect no difference in protective immunity between the two groups. Since the same PCR procedures were used throughout, we may assume that the second diagnosis was correct, too, even though additional verification using DNA sequencing would have been desirable.

On the other hand, we note that even in the control group the infection fatality rate was only about 0.12%, which agrees with similar estimates by Brown [2] and Ioannidis et al. [3]. This overall remarkably low number can be ascribed to cross-immunity conferred by regular respiratory coronavirus strains [4,5]. Overall, the study by Abu-Raddad et al. confirms that natural immunity can be relied upon to protect the populace from severe recurrent COVID-19 disease, and that vaccination is unnecessary.


  1. Of 353,326 individuals who had previously been diagnosed with COVID, 87,547 were excluded from the study because they had been vaccinated. The study provides no information on incidence and severity of reinfections in this group.


  1. Abu-Raddad, L.J. et al. (2021) Severity of SARS-CoV-2 Reinfections as Compared with Primary Infections. N. Engl. J. Med. 385:2487-2489
  2. Brown, R.B. (2020) Public health lessons learned from biases in coronavirus mortality overestimation. Disaster Med. Public Health Prep. pp. 1-24
  3. Ioannidis, J.P.A. (2020) Infection fatality rate of COVID-19 inferred from seroprevalence data. Bull. World Health Organ. p. BLT.20.265892
  4. Bhakdi, S. et al. (2021) Letter to Physicians: Four New Scientific Discoveries Regarding COVID-19 Immunity and Vaccines—Implications for Safety and Efficacy.
  5. Palmer, M. and Bhakdi, S. (2021) Rebuttal to Geert vanden Bossche’s “Response to Dr. Bhakdi”.
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