New data on duration of protection afforded by natural SARS-CoV-2 infection

In a recent study posted to the medRxiv* preprint server, researchers investigated the immune protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) natural reinfection.

The currently available coronavirus disease 2019 (COVID-19) vaccines played a critical role in curbing the number of COVID-19-related hospitalizations and mortalities. However, their waning immune efficacy has led to an increased number of SARS-CoV-2 reinfections which has further limited their usefulness in curbing the COVID-19 pandemic.

Study: Duration of immune protection of SARS-CoV-2 natural infection against reinfection in Qatar. Image Credit: NIAID

About the study

In the present study, researchers explored the duration of protection provided by natural infection, as well as the impact of viral immune evasion on this duration and the robustness of the protection against severe SARS-CoV-2 infection.

The team performed a retrospective study using randomized target trials using COVID-19 data related to laboratory testing, COVID-19 vaccination, and associated hospitalization and death. In this study, all SARS-CoV-2 data from healthcare facilities, including PCR tests and rapid antigen tests, was collected.

The team compared the incidence of either SARS-CoV-2 infection or severe, critical, or fatal COVID-19 disease among the primary-infection cohort comprising individuals reporting a primary COVID-19 infection diagnosed before the vaccination, a control cohort including persons who are infection-naive, and the infection-naive cohort including unvaccinated individuals.

Individuals belonging to the primary-infection cohort were matched to individuals from the infection-naive cohort according to gender, age group, nationality, and the number of comorbidities. Individual matching was also performed as per the calendar week when the SARS-CoV-2 testing was conducted. The monitoring of each matched pair began when an individual from the primary-infection cohort reached 90 days following diagnosis.

The pre-Omicron reinfection study estimated the efficacy of a pre-Omicron primary infection against pre-Omicron reinfection, and the Omicron reinfection study assessed the efficacy of a pre-Omicron primary infection against an Omicron lineage reinfection. Furthermore, the SARS-CoV-2 severity reinfection study evaluated the efficacy of a primary SARS-CoV-2 infection against either severe, critical, or fatal reinfection, irrespective of the causative variant. Any individual who reported a primary infection from 28 February 2020 to 30 November 2021 was included in the primary-infection group. Any individual who tested SARS-CoV-2-negative throughout this period was included in the infection-naive group. The study outcomes included an infection diagnosis and the occurrence of either severe, critical, or fatal SARS-CoV-2 infection. 

Results

The study results showed that among the 1,806 reinfections detected in the primary-infection group throughout the follow-up period of the pre-Omicron reinfection study, six individuals experienced severe COVID-19 while one patient succumbed to the infection. The infection-naive cohort included 11,957 cases, among which 297 cases experienced severe, 19 critical, and 12 fatal COVID-19. The primary-infection and the infection-naive cohorts reported 1.7% and 9.6% cumulative infection incidence in 15 months post-primary infection.

The team observed that pre-Omicron primary infection was 85.5% effective against pre-Omicron reinfection. Moreover, this efficacy increased to 90.5% in the seventh month following the primary infection, which subsequently decreased to almost 70% by the 16th-month post-primary infection. The team also noted that the efficacy reduced to 50% and less than 10% in the 22nd and the 32nd month. Furthermore, the efficacy of the primary pre-Omicron infection was 98.0% against severe, critical, and fatal COVID-19 caused by reinfection with a pre-Omicron variant.

The Omicron reinfection study showed that among the 7995 reinfections found in the primary-infection cohort throughout the follow-up period, five individuals experienced severe COVID-19. The infection-naive cohort included 12,230 cases, among which 26 cases experienced severe, seven critical, and five fatal COVID-19 infections. The primary infection and the infection-naive cohorts reported 6.8% and 10.4% cumulative infection incidence in 165 days after the beginning of the follow-up period.

The team observed that pre-Omicron primary infection was 38.1% effective against Omicron reinfection. Moreover, the efficacy was almost 60% against Omicron reinfection for individuals who tested positive for a primary infection between 1 June 2021 and 30 November 2021. However, this effectiveness was 17.0% and 50% for individuals who reported primary infection between 1 December 2020 and 28 February 2021 and before 31 August 2020, respectively. Furthermore, the efficacy of the primary pre-Omicron infection was 88.6% against severe, critical, and fatal COVID-19 caused by Omicron reinfection.

The COVID-19 severity reinfection study showed that among the 7082 reinfections in the primary-infection cohort, nine and one individual had severe and fatal COVID-19, respectively. The infection-naive cohort included 21,645 cases, among which 315 experienced severe, 25 critical, and 18 fatal COVID-19. The primary-infection and the infection-naive cohorts reported 0.0% and 0.21% cumulative infection incidence 15 months post-primary infection. The efficacy of the primary infection was 97.3% against severe, critical, and fatal COVID-19 caused by any variant.

Overall, the study findings showed that natural SARS-CoV-2 infection provided robust protection against reinfection for over 14 months after the primary infection.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Bhavana Kunkalikar

Bhavana Kunkalikar is a medical writer based in Goa, India. Her academic background is in Pharmaceutical sciences and she holds a Bachelor's degree in Pharmacy. Her educational background allowed her to foster an interest in anatomical and physiological sciences. Her college project work based on ‘The manifestations and causes of sickle cell anemia’ formed the stepping stone to a life-long fascination with human pathophysiology.