COVID-19 vaccination reduces risk and duration of SARS-CoV-2 infection

In a study conducted at the European Institute of Oncology, Italy, scientists have found that the risk and duration of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection reduced significantly after coronavirus disease 2019 (COVID-19) vaccination.

Study: Lower probability and shorter duration of infections after Covid-19 vaccine correlate with anti-SARS-CoV-2 circulating IgGs. Image Credit: Andy Dean Photography/ ShutterstockStudy: Lower probability and shorter duration of infections after Covid-19 vaccine correlate with anti-SARS-CoV-2 circulating IgGs. Image Credit: Andy Dean Photography/ Shutterstock

A preprint version of the study is available on the medRxiv* server while the article undergoes peer review.


The COVID-19 pandemic has caused more than 230 million infections and 4.75 million deaths worldwide. After the introduction of vaccines, a reduction in infection and mortality rates has been observed in many countries across the globe. According to the World Health Organization (WHO) report, around 5.9 billion vaccine doses have been administered already.

As documented in the literature, COVID-19 vaccines induce robust humoral and cellular immunity and protect against SARS-CoV-2 infection and symptomatic disease. Regarding the duration of vaccine immunity, studies have indicated that the effectiveness of vaccines remains stable for at least six months. However, recent evidence on vaccine breakthrough infections has highlighted the possibility of waning vaccine immunity with time.

In the current study, the scientists have assessed the risk and duration of infection in vaccinated and unvaccinated healthcare workers. Moreover, they have determined the correlation between the risk of infection and the level of humoral immunity.

Study design

The study was conducted on the healthcare workers, support staff, administrative staff, and research personnel working at the European Institute of Oncology, Italy. All participants were monitored for 14 months.

During the pre-vaccination period (April 2020 to January 2021), a total of 1493 participants were tested for SARS-CoV-2 infection by reverse transcription-polymerase chain reaction (RTPCR). The serum levels of anti-spike receptor-binding domain (RBD) IgG antibodies were also measured by enzyme-linked immunosorbent assay (ELISA).

Similarly, a total of 2029 participants, including the participants from the pre-vaccination period, were subsequently vaccinated and monitored during the post-vaccination period.

Important observations

Among 2029 vaccinated participants, 1818 received the mRNA-based COVID-19 vaccine developed by Pfizer/BioNTech, and 133 received the adenoviral vector-based vaccine developed by Oxford/AstraZeneca. Except for 48 participants from each vaccine group, all were fully immunized with two vaccine doses.

Risk of infection before vaccination

Among 1493 unvaccinated participants, around 18% developed SARS-CoV-2 infection. Compared to non-medical staff, medical personnel, including nurses and physicians, showed significantly higher susceptibility to develop the infection. Importantly, seropositive participants showed a 66% lower risk of developing an infection.

Among 266 participants who became SARS-CoV-2 positive during the initial phase of the study, eight developed reinfection (3%). Refection occurred in five participants more than 60 days after the previous infection. Overall, the frequencies of reinfection among seropositive and seronegative participants were 9% and 25%, respectively.   

Risk of infection after vaccination

A significantly high serum level of anti-RBD antibody was detected in almost all vaccinated participants one week after vaccination. Only 1.9% of participants were non-responsive to vaccination. An inverse correlation was observed between antibody level and age. Although it remained detectable four months after vaccination, a gradual decline in antibody level was observed over time.

Among 2029 vaccinated participants, only 30 developed infections during the study period (1.5%). This indicates a significantly lower risk of infection in vaccinated participants compared to unvaccinated participants. The average duration between the last vaccination and breakthrough infection was 55 days.

The average duration of infection in vaccinated participants was two days, which was significantly shorter than the duration of natural infection (16 days) or reinfection (11 days) in unvaccinated participants. Moreover, all participants with vaccine breakthrough infections remained asymptomatic or mildly symptomatic. Overall, these findings further highlight the effectiveness of vaccination (shorter viral shedding duration and protection against severe disease).

Correlation between antibody level and infection

A significantly lower frequency of infection (1.4%) was observed in seropositive vaccinated participants compared to that in seropositive unvaccinated participants (9%) and seronegative vaccine non-responders (5.7%).    

Among 30 participants with vaccine breakthrough infections, 27 had lower levels of anti-RBD antibodies, and three were unresponsive to vaccination. Overall, these findings indicate that vaccinated individuals with high antibody titers are less susceptible to develop breakthrough infections.

Study significance

The study highlights the significance of COVID-19 vaccination in reducing the risk and duration of infection. However, a small percentage of vaccine breakthrough cases observed in the study indicate that even vaccinated individuals should utilize non-pharmaceutical control measures (mask-wearing, social distancing, etc.) for additional protection.     

*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. Sanchari Sinha Dutta

Written by

Dr. Sanchari Sinha Dutta

Dr. Sanchari Sinha Dutta is a science communicator who believes in spreading the power of science in every corner of the world. She has a Bachelor of Science (B.Sc.) degree and a Master's of Science (M.Sc.) in biology and human physiology. Following her Master's degree, Sanchari went on to study a Ph.D. in human physiology. She has authored more than 10 original research articles, all of which have been published in world renowned international journals.


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  1. Atilda Mosham Atilda Mosham Australia says:

    Recent official UK Government document clearly demonstrates that fully vaccinated are 5.3 times more likely to die than the unvaccinated when infected with the Delta variant. Look for "SARS-CoV-2 variants of concern and variants under investigation in England
    Technical briefing 20" go to statistics on page 18 and 19.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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