Impact of prior SARS-CoV-2 infection and mRNA vaccination on contagiousness and infection risk

In a recent study published in Nature Communications, researchers examine the effect of immunity conferred by previous severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and messenger ribonucleic acid (mRNA) coronavirus disease 2019 (COVID-19) vaccination on the infectiousness, susceptibility, and transmission of SARS-CoV-2.

Study: Effect of SARS-CoV-2 prior infection and mRNA vaccination on contagiousness and susceptibility to infection. Image Credit: Kateryna Kon / Shutterstock.com

Background

COVID-19 continues to burden healthcare systems throughout the globe due to its long-term complications. Furthermore, the high transmission rates of SARS-CoV-2 in both human and animal populations have allowed this virus to mutate rapidly, thereby resulting in consecutive waves of infection due to the emergence of novel variants with immune-evasive abilities.

Understanding how immunity against SARS-CoV-2 impacts its transmission and evolution is essential in deciphering how the virus spreads. Previous SARS-CoV-2 infection and various COVID-19 vaccines can confer immunity against SARS-CoV-2. While mRNA COVID-19 vaccines are effective in preventing infection for several months following vaccination, immunity rapidly wanes, thus necessitating the need for multiple booster vaccination doses.

While SARS-CoV-2 infection may confer stronger immunity than mRNA vaccines, how this type of immunity differentially impacts infectiousness, susceptibility, and SARS-CoV-2 transmission remains unclear.

About the study

In the current study, over 50,000 SARS-CoV-2 positive cases and over 100,000 contacts were included to understand the impact of immune status on the secondary attack rate, which is an effective measure of viral transmission for four SARS-CoV-2 variants. About 73% of index cases included adults between 18 and 64 years of age, 22% were children, and the remaining 5% were adults over the age of 65.

The study considered vaccination status, as well as previous natural SARS-CoV-2 infection of the index case and contacts while adjusting for factors such as the tendency to get tested for SARS-CoV-2, contact settings, health and social factors, and demographic characteristics. An individual who was in contact with an index case was considered infected if they tested positive for COVID-19 within 10 days.

Data were collected between February 2020 and February 2022, which reflects the time between the first test reported and the discontinuation of contact reporting in Geneva. These data were obtained from the Actionable Register of Geneva Outpatients and inpatients with SARS-CoV-2 (ARGOS) register, which contains information from all COVID-19-positive individuals at baseline and follow-up, demographic characteristics, and all individuals they have been in contact with.

The secondary attack rate or transmission was calculated by dividing the number of infected contacts by the total number of contacts who were reported susceptible by the index cases. Immune status was characterized based on the vaccination status and history of a previous SARS-CoV-2 infection.

The immunity effect estimation was controlled for a wide range of factors, including demographic characteristics, socioeconomic conditions, body mass index, and neighborhood of residence. The analysis was also categorized according to the age group of zero to 17 years, 18 to 64 years, and older than 65 years. The periods of dominance of the EU1, Alpha, Delta, and Omicron variants were used to determine the SARS-CoV-2 variant of infection.

Study findings

The immunity conferred by vaccination and previous SARS-CoV-2 infection was more effective in protecting individuals from infection than reducing infectiousness in individuals with COVID-19. Natural SARS-CoV-2 infection also conferred stronger immunity than vaccination, whereas recent or older SARS-CoV-2 infections conferred the same level of immunity as hybrid immunity.

Susceptibility to infection was more affected by the emergence of novel SARS-CoV-2 variants and time as compared to reductions in infectiousness due to vaccination, which was observed to be less sensitive to newly emergent variants and waned more slowly with time.

These observations highlight the role of mRNA vaccines in reducing the infectiousness of SARS-CoV-2-positive individuals. Disease-mitigating interventions such as proper indoor ventilation and the use of face masks also mitigated the transmission of SARS-CoV-2.

Conclusions

The study findings indicate that while natural SARS-CoV-2 infection granted stronger immunity than COVID-19 vaccination or hybrid immunity, the immunity granted by mRNA vaccines played a significant role in reducing the infectiousness and contributed to decreasing the transmission of SARS-CoV-2.

Written by

Dr. Chinta Sidharthan

Chinta Sidharthan is a writer based in Bangalore, India. Her academic background is in evolutionary biology and genetics, and she has extensive experience in scientific research, teaching, science writing, and herpetology. Chinta holds a Ph.D. in evolutionary biology from the Indian Institute of Science and is passionate about science education, writing, animals, wildlife, and conservation. For her doctoral research, she explored the origins and diversification of blindsnakes in India, as a part of which she did extensive fieldwork in the jungles of southern India. She has received the Canadian Governor General’s bronze medal and Bangalore University gold medal for academic excellence and published her research in high-impact journals.