Question & Answer

If I've already had COVID-19 am I immune to re-infection?
Click image to download
  • Here we discuss reinfection rates in people who were not vaccinated.
  • Most of the systematic reviews we found reported on studies when the original types of SARS-CoV-2 (the virus that causes COVID-19) were circulating rather than newer variants.
  • These studies reported that a previous infection was between 81% and 97% effective at preventing a subsequent infection. Reinfections occurred in 0% to 3% of people who had previously had COVID-19.
  • True reinfection can only be accurately confirmed by using a technique called whole genome sequencing(WGS), to make sure that the first and second infections are separate infections.  Most studies did not use WGS but relied on PCR tests, which can be less reliable for this kind of information.
  • Reviews of the evidence have found that different variants of virus confer differing levels of protection against other variants.
  • Some studies reported on the severity of second COVID-19 infections. One review found that overall, the rate of serious illness in someone with prior infection was 0.02% and that even after more than three months since the first infection, this did not rise above 0.1%. Another review found that any prior infection was highly protective against severe COVID-19 or death during the Omicron period.
  • The length of time that people were followed varied between the studies. The shortest follow-up was two weeks, and the longest was 20 months.
  • The duration of protection from infection from the original strains of SARS-CoV-2 was reported in three reviews (here, here and here) as lasting for at least eight months and at least 20 months during Delta. The evidence on the duration of immunity during the circulation of other variants was of low certainty.
  • Immune memory can be extremely long lived, e.g., more than 60 years after smallpox vaccination and 90 years after infection with influenza. Some parts of immune memory from the first SARS-CoV epidemic in 2003 have been detected 17 years after infection. We need studies that follow people for a longer period to find out how long-term immunity, including immune memory, protects us against COVID-19 infection, serious illness or death.

Things to Remember

Reviewers

  • Lead Researcher: Dr. Paula Byrne, Senior Post-doctoral Researcher, Evidence Synthesis Ireland & HRB-Trials Methodology Research Network, College of Medicine, Nursing and Health Sciences, University of Galway
  • Reviewed by: Prof Declan Devane, Professor of Health Research Methodology, Deputy Dean, College of Medicine, Nursing and Health Sciences, University of Galway,
  • Scientific Director, HRB-Trials Methodology Research Network Director, Evidence Synthesis Ireland. Director, Cochrane Ireland
  • Topic advisor: Dr. Frank Moriarty, Senior Lecturer in Pharmacy at the School of Pharmacy and Biomolecular Sciences, Royal College of Surgeons in Ireland and visiting research fellow at The Irish Longitudinal Study on Ageing (TILDA).
  • Public and Patient advisor: Anne Daly, Public and Patient Involvement in Research (PPI) advisor, PPI Ignite, University of Galway.
  • Journalist Advisor: Dr. Claire O’Connell, PhD in cell biology, Masters in Science Communication. Contributor to The Irish Times, writing about health, science and innovation.

Conflict of Interest Statement: The authors have no financial conflicts of interest for this health claim summary. Paula Byrne previously was co-reviewer on several review and updates of immunity to SARS-CoV-2 for the Health Information and Quality Authority and is co-author on three published papers on related topics.

View All