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In this Journal, Salvagno GL and colleagues supposed Omicron variant patients might exhale more viral particles than previous variant patients based on the higher nasopharyngeal viral load in Omicron infections.
Unfortunately, they did not directly compare the viral load of exhaled breath from COVID-19 patients. In this study, the amount of SARS-CoV-2 exhaled by Omicron and Delta patients was compared. Our findings may help to explain why the Omicron variant is more transmissibility than the Delta variant.
It was reported that the effective reproduction number (Re) and basic reproduction number (R0) of the Omicron variant elicited 3.8 and 2.5 times higher transmissibility than the Delta variant, respectively.
However, the reasons driving the increased transmissibility of the Omicron variant are still not well understood, and the viral load in exhaled breath of patients with Omicron and Delta has not been compared.
Here, serial exhaled breath specimens and nasopharyngeal swabs were taken at 1, 3, 5, 7, 9, 11, and 13 days post hospitalization (dph) from 5 Delta and 8 Omicron patients. The Delta patients were selected from the group of COVID-19 patients in our previous study.
Exhaled breath specimens were collected for 5 min from all the patients by using a BioScreen device (Dingblue Technology Co., LTD, Beijing, China). SARS-CoV-2 RNA was extracted by using a viral RNA extraction kit (Tguide, TIANGEN) and detected by a SARS-CoV-2 nucleic acid detection kit (PCR-Fluorescence Probing) (Sansure Biotech, Hunan, China), targeting both the N and ORF1ab genes. A standard curve was fitted using a series of 10-fold dilutions of a standard plasmid DNA containing N genes of SARS-CoV-2. The numbers of viral RNA copies in the samples were estimated from the measured cycle threshold (Ct) values.
As shown in Fig. 1A, the Omicron variant patients exhaled more SARS-CoV-2 virions than the Delta variant patients at 1, 3, 5, 7 and 9 dph, and it showed significantly higher at 3 and 5 dph (*, P < 0.05; **, P < 0.01). Both the Omicron and Delta variant patients exhaled ten million SARS-CoV-2 particles per hour (107 viral RNA copies) at 1 dph (Fig. 1A). At the first week of hospitalization, the overall breath emission rate of Delta variant patients was from 105.33 to 107.15copies per hour and Omicron variant patients was from 105.60 to 107.67copies per hour (Fig. 1B). The average breath emission rate of Omicron variant patients (107.3copies/h) was higher than the Delta variant patients (106.6copies/h) at the first week of hospitalization.
To explore whether the higher viral load exhaled by Omicron patients is related to an increased nasopharyngeal viral load. The viral loads in nasopharyngeal swabs of the Delta and Omicron variants patients were also measured. The Omicron variant patients did not contain higher nasopharyngeal viral load than the Delta variant patients (Fig. 1C). Furthermore, we also analyzed the trend of viral load in nasopharyngeal swabs and exhaled breath from patients with Delta or Omicron, respectively. The viral loads in nasopharyngeal samples from patients with Delta or Omicron declined with increasing duration of illness after hospitalization, and the exhaled breath specimens had the similar variation trend with nasopharyngeal samples (Fig. 1D).
It was reported that Omicron variant lead to more significant immune escape compared with Delta variant,
which might one reason for the increased transmissibility of Omicron. In this study, we found Omicron patients exhaled more virus particles than Delta patients at the first week of hospitalization, which provided another reason for the increased transmissibility. We also found that the Omicron and Delta variants had similar viral load in nasopharyngeal swab samples as previously reported.
but might be by other factors, such as enhanced immune escape and virus shedding ability. The viral load in exhaled breath had the similar variation trend with nasopharyngeal specimens from COVID-19 patients (Delta or Omicron), suggesting that COVID-19 patients who infected with the same SARS-CoV-2 strain and had higher nasopharyngeal viral load might exhibit higher virus excretion.
In conclusion, more SARS-CoV-2 particles were exhaled by the Omicron patients than the Delta patients, which provide a direct evidence for the enhanced transmissibility of the Omicron variant.
Regarding the ethical requirements of human subjects, all specimen collections were obtained with the informed consent of the patients. None of the specimens recorded patient identifiers. The Ethics and Scientific Review Committee of Sierra Leone approved the study.
Declaration of Competing Interest
The authors declare no competing interests.
This research was supported by the National Key R&D Program of China ( 2021YFC2301804 ), the Biosafety Special Program (No. 19SWAQ 13 ) and the National Natural Science Foundation of China ( 82150202 and 32000134 ).
The authors would like to give our sincere thanks to the hospital staff that supported us in recruiting the patients and collecting the specimens. The authors are most grateful to all SARS-CoV-2 patients for participating in this study.
De Nitto S.
SARS-CoV-2 Omicron infection is associated with high nasopharyngeal viral load.