SARS-CoV-2 immunity due to prior infection or vaccination is similar, study says

A new study published on the medRxiv* preprint server finds that the rate of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) reinfection is low following one episode of the infection. Breakthrough infections were also found to be similarly rare after vaccination with a messenger ribonucleic acid (mRNA) vaccine, say the researchers.

Study: Incidence of Severe Acute Respiratory Syndrome Coronavirus-2 infection among previously infected or vaccinated employees. Image Credit: nobeastsofierce /


SARS-CoV-2, which was originally detected in Wuhan, China in December 2019, is the virus responsible for the devastating coronavirus disease 2019 (COVID-19). The impact of COVID-19, along with the comprehensive restrictions that have been placed on social interactions, has forced businesses and schools to close. As a result, millions of people around the world became unemployed, which has caused the global economy to suffer tremendously.

The continued urgency of the COVID-19 pandemic has led to unprecedented efforts to develop effective antivirals and vaccines to combat this virus. These efforts have led to the development of several safe and high efficacious vaccines that have since been approved for public use in many countries around the world. These vaccines promise a respite from the unending overload on healthcare staff and facilities.

The earliest vaccines to gain emergency use authorization included the mRNA vaccines from Pfizer/BioNTech and Moderna. The mRNA vaccines deliver viral genetic material encoding for the immunogenic viral spike (S) glycoprotein, which enters the cells of the vaccine recipient to activate the immune system.

How was the study performed?

The current study examined how vaccination compares with natural SARS-CoV-2 infection rates in its ability to reduce the rate of infection. This research was conducted by Curative, which is a company that specializes in SARS-CoV-2 testing. To this end, Curative began to test its employees using a reverse transcriptase-polymerase chain reaction (RT -PCR) method that was authorized by the United States Food and Drug Administration (FDA).

Daily testing was combined with an automated database that was implemented on May 8, 2020. To date, Curative is continuing to test its employees daily, even after vaccination campaigns began.

Among the three different groups of employees that were included in the current study, one was defined as the naïve unvaccinated group. This group, which consisted of over 4,300 records, remained applicable until December 15, 2020, which is when vaccination was offered to all employees.

The second group included unvaccinated individuals with a history of prior infection, which included 254 records. The third group, which consisted of almost 740 records, comprised vaccinated people from December 15, 2020, to July 1, 2021.

Any individual who had two positive RT-PCR tests was defined as having SARS-CoV-2 infection. The comparisons between the three groups were on the basis of the incidence rate ratio (IRR), defined as “the ratio of confirmed COVID-19 cases per 100 person-years.”

Did the incidence decrease after infection or vaccination?

Within the naïve group, approximately 26 new cases were reported per 100 person-years. In the second group, which consisted of previously infected and unvaccinated individuals, and the third group of vaccinated individuals, there were zero and 1.6 cases, respectively, per 100 person-years.

This implies an incident rate ratio (IRR) of zero following natural infection with the virus. That is, when the incidence among those with a history of infection (zero) is compared to that among the naïve individuals (26), the former have no risk of reinfection.

Conversely, the IRR was 0.06 when the incidence among vaccinated (1.6) and naïve (26) individuals were compared. Therefore, the risk of SARS-CoV-2 infection following vaccination was less than one-tenth of the latter. Finally, vaccine recipients had no increased risk of breakthrough infection with the virus, as compared to those with a history of prior infection.  

What are the implications?

The current study demonstrates that both prior infection and receipt of an mRNA vaccine confer a high degree of immunity against reinfection or breakthrough infections, respectively. This observation has also been described in other stories that reported the equivalent and high protective efficacy of vaccination with the BNT162b2 or ChAdOx1 nCOV-19 vaccines and prior infection with the virus.

This protection likely operates through both humoral and cellular arms of the adaptive immune response. The virus S protein and other antigens elicit specific and durable T cell responses targeting these epitopes. The diversity of the T cell response allows a breadth of protection against the ancestral SARS-CoV-2, as well as key variants of concern such as the B.1.1.7, B.1.351, and P.1 lineages.

Moreover, memory B cells have been detected that provide strong antibody-mediated immunity over the long term, up to six months after infection.

Combined with earlier studies, our findings should provide increased confidence that those previously infected are at very low risk for repeat infection."

A possibility to be ruled out by further study is the rise of new variants, which could be an alternative explanation for the low detection of the virus in this group caused by mutations that prevent recognition of the viral genetic material by the RT-PCR primer sequences.

*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.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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