Comparison of Dose-Response Relationships for Two Isolates of SARS-CoV-2 in a Nonhuman Primate Model of Inhalational COVID-19

Abstract

Background: As the COVID-19 pandemic has progressed, numerous variants of SARS-CoV-2 have arisen, with several displaying increased transmissibility. Methods: The present study compared dose-response relationships and disease presentation in nonhuman primates infected with aerosols containing an isolate of the Gamma variant of SARS-CoV-2 to the results of our previous study with the earlier WA-1 isolate of SARS-CoV-2. Results: Disease in Gamma-infected animals was mild, characterized by dose-dependent fever and oronasal shedding of virus. Differences were observed in shedding in the upper respiratory tract between Gamma- and WA-1-infected animals that have the potential to influence disease transmission. Specifically, the estimated median doses for shedding of viral RNA or infectious virus in nasal swabs were approximately 10-fold lower for the Gamma variant than the WA-1 isolate. Given that the median doses for fever were similar, this suggests that there is a greater difference between the median doses for viral shedding and fever for Gamma than for WA-1 and potentially an increased range of doses for Gamma over which asymptomatic shedding and disease transmission are possible. Conclusions: These results complement those of previous studies, which suggested that differences in exposure dose may help to explain the range of clinical disease presentations observed in individuals with COVID-19, highlighting the importance of public health measures designed to limit exposure dose, such as masking and social distancing. The dose-response data provided by this study are important to inform disease transmission and hazard modeling, as well as to inform dose selection in future studies examining the efficacy of therapeutics and vaccines in animal models of inhalational COVID-19.

Keywords: COVID-19; aerosol; dose–response; inhalation; nonhuman primates; transmission.

FIG. 1.

Representative temperature profiles. Temperature profiles are shown for an animal, which developed a transient fever at 29 hours postexposure (A), and for an animal that did not develop fever postexposure (B). The deposited doses for (A) and (B) were 637.3 TCID₅₀ (2.80 log₁₀ TCID₅₀) and 90.4 TCID₅₀ (1.96 log₁₀ TCID₅₀), respectively. The dips in temperature observable postexposure coincide with administration of anesthesia before blood collection. TCID₅₀, median tissue culture infectious doses.

FIG. 2.

Shedding of viral RNA and infectious virus in nasal and oral swabs from animals exposed to SARS-CoV-2 Gamma. Levels of viral RNA (A) and infectious virus (B) in swab samples, expressed as log₁₀ RNA copies/mL and log₁₀ TCID₅₀/mL, respectively, are shown for animals exposed to SARS-CoV-2 Gamma. Animals are grouped by disease presentation, specifically those with fever and a neutralizing titer (i.e., positive PRNT₅₀), those with seroconversion but without fever, and those with neither response. PRNT, plaque reduction neutralization test.

FIG. 3.

Shedding of viral RNA and infectious virus in nasal and oral swabs from animals exposed to SARS-CoV-2 WA-1. Levels of viral RNA (A) and infectious virus (B) in swab samples, expressed as log₁₀ RNA copies/mL and log₁₀ TCID₅₀/mL, respectively, are shown for animals exposed to SARS-CoV-2 WA-1. Animals are grouped by disease presentation, specifically those with fever and a neutralizing titer (i.e., positive PRNT₅₀), those with seroconversion but without fever, and those with neither response.

FIG. 4.

Exhaled particle concentration profile and particle size distribution. The particle concentration profile during a single collection period is shown. Particle concentrations were low for the majority of the recording period. However, the animal sneezed ∼3.5 minutes into the collection period, resulting in a large spike in particle concentration. The particle size distribution associated with the sneeze measured by an APS is also shown. Unfortunately, this event occurred during the preexposure period, and so no virus was detected. However, these data provide confidence that higher particle emission rates are able to be detected in the system. APS, aerodynamic particle sizer.

FIG. 5.

Exhaled particle emission rates. (A) Exhaled particle emission rates are shown for uninfected animals that did not seroconvert, develop fever, or shed viral RNA in the exhaled breath. Emission rates for animals exposed to SARS-CoV-2 Gamma (gray circles) and WA-1 (black circles) are shown. (B) Exhaled particle emission rates are shown for animals with PCR-positive exhaled breath samples. Emission rates for the four animals infected with Gamma (gray circles) and the one animal infected with WA-1 (black circles) are shown. For animals infected with Gamma, the particle emission rates did not significantly change over the postexposure period (p = 0.9669) when compared using a Friedman test with Dunn's multiple comparisons posttest. Similarly, the particle emission rates were not different at any time point postexposure between uninfected animals (A) and those with PCR-positive exhaled breath samples (B) (p = 0.41 when compared using a Kruskal–Wallis test).

FIG. 6.

Exhaled particle emission rates as a function of viral RNA load in nasal swabs for animals infected with SARS-CoV-2 Gamma. No relationship was observed between exhaled particle counts and the viral RNA load detected in nasal swab samples across all animals (black and gray circles; r² = 0.006; n = 39 samples). Similarly, no relationship is observed if this analysis is restricted only to samples from animals with fever (black circles; r² = 0.008; n = 21 samples).