Viral culture and immunofluorescence for the detection of SARS-CoV-2 infectivity in RT-PCR positive respiratory samples

• Published in: Journal of clinical virology Vol. 152; p. 105167
• Main Authors: Berengua, Carla, López, Marina, Esteban, Montserrat, Marín, Pilar, Ramos, Paula, Cuerpo, Margarita del, Gich, Ignasi, Navarro, Ferran, Miró, Elisenda, Rabella, Núria
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.07.2022
• Subjects: COVID-19; COVID-19 - diagnosis; COVID-19 Testing; Fluorescent Antibody Technique; Humans; Infectivity; Paper from the 21st ESCV meeting; Reverse Transcriptase Polymerase Chain Reaction; SARS-CoV-2; VERO E6 cells; Viral culture; Viral isolation; COVID-19; VERO E6 cells; Viral culture; SARS-CoV-2; Viral isolation; Infectivity
• ISSN: 1386-6532; 1873-5967; 1873-5967
• DOI: 10.1016/j.jcv.2022.105167

•Viral isolation was successful in 58% of respiratory samples with positive RT-PCR.•SARS-CoV-2 isolation is correlated with days of symptoms and RT-PCR Ct value.•SARS-CoV-2 infectivity lasts no more than 14 days in immunocompetent individuals.•Ct value <22 always indicates infectivity. Knowing how long SARS-CoV-2-positive individuals can remain infective is crucial for the design of infection prevention and control strategies. Viral culture is the gold standard for detecting an active-replicative virus and evaluating its infectious potential. To assess the correlation of SARS-CoV-2 infectivity with the number of days from symptom onset and the Ct value, using culture as a reference method. Also, to describe a detailed protocol for SARS-CoV-2 culture and immunofluorescence confirmation based on our experience with other respiratory viruses. 100 consecutive respiratory samples positive for SARS-CoV-2 by RT-PCR from different subjects were inoculated into VERO E6 cells. Viral isolation was successful in 58% of samples. The median number of days from symptom onset for culture-positive samples was 2, and 15 for culture-negative samples. Six positive cultures were obtained in patients ≥14 days after symptom onset, all of whom were immunocompromised or with severe COVID-19. The mean Ct value was 12.64 units higher in culture-negative than in culture-positive samples. The probability of successfully isolating SARS-CoV-2 in samples with a Ct value <22 was 100%, decreasing to 3.1% when >27. Our findings show a significant positive correlation between the probability of isolating SARS-CoV-2 in culture, fewer days of symptoms and a lower RT-PCR Ct value. SARS-CoV-2 infectivity lasts no more than 14 days from symptom onset in immunocompetent individuals. In contrast, in immunocompromised patients or those with severe COVID-19 infectivity may remain after 14 days. Ct value <22 always indicates infectivity.