SARS-CoV-2 inactivated vaccine (Vero cells) shows good safety in repeated administration toxicity test of Sprague Dawley rats

Abstract

The outbreak of COVID-19 has posed a serious threat to global public health. Vaccination may be the most effective way to prevent and control the spread of the virus. The safety of vaccines is the focus of preclinical research, and the repeated dose toxicity test is the key safety test to evaluate the vaccine before clinical trials. The purpose of this study was (i) to observe the toxicity and severity of an inactivated SARS-CoV-2 vaccine (Vero cells) in rodent Sprague Dawley rats after multiple intramuscular injections under the premise of Good Laboratory Practice principles and (ii) to provide a basis for the formulation of a clinical trial scheme. The results showed that all animals in the experimental group were in good condition, no regular changes related to the vaccine were found in the detection of various toxicological indexes, and no noticeable stimulating reaction related to the vaccine was found in the injected local tissues. The neutralizing antibodies in the low- and high-dose vaccine groups began to appear 14 days after the last administration. In the negative control group, no neutralizing antibodies were observed from the administration period to the recovery period. Therefore, the repeated administration toxicity test of the inactivated SARS-CoV-2 vaccine (Vero cells) in Sprague Dawley rats showed no obvious toxic reaction. It was preliminarily confirmed that the vaccine can stimulate production of neutralizing antibodies and is safe in Sprague Dawley rats.

Keywords: COVID-19; Safety; The inactivated vaccine; The repeated administration toxicity.

Fig. 1

The results of the CD4⁺ and CD8⁺ T cell counts. Compared with the negative control group, there was no significant difference in the proportion of peripheral blood CD4⁺ and CD8⁺ T cells in the solvent control group and the low- and high-dose groups. Furthermore, the CD4⁺/CD8⁺ cell ratio of each group showed no statistical difference with the negative control group.

Fig. 2

The results of the neutralizing antibodies. The neutralizing antibodies in all groups were negative before grouping. At 14 days after the first administration, the neutralizing antibodies in the low-dose vaccine satellite group and the high-dose vaccine satellite group began to appear, with average titers of 1:5.2 and 1:17.6, respectively; at 14 days after the second administration, the levels of neutralizing antibodies of the two groups gradually increased, with average titers of 1:80 and 1:108.8, respectively; and at 14 days after the third administration, the levels of neutralizing antibodies of the two groups continued to rise, with average titers of 1:128 and 1:128, respectively. The neutralizing antibody levels of the two groups remained at 1:128 until 14 days after the fourth administration.

Fig. 3

The results of the histopathological examination. A: At 3 days after the last dose, a small number of inflammatory cells were found in the myocardium of two animals in the low-dose vaccine group, 14 days after the last dose, and one animal in the solvent control group, one in the low-dose group, and one in the high-dose vaccine group had the same pathological symptoms; B: At 3 days after the last dose, there were two small foci of inflammatory necrosis in the liver parenchyma and sink area, mainly including lymphocytes and emptying hepatocytes. This pathological symptom also appeared 14 days after the last dose in two animals in the negative control group and the solvent control group; C: Two animals in the negative control group also developed glomerular atrophy or increased glomerular capillaries, but its structure was clear and complete, and there was no obvious structural cell necrosis. Also in the negative control group, two animals showed a large amount of exudate and a small amount of monocytes and local glomerular atrophy with increased glomerular capillaries; D,E: At 3 days after the last dose, pulmonary pathology showed that one or two animals in each group showed local pulmonary interstitial thickening, alveolar epithelial cell proliferation, shedding and varying degrees of vascular and alveolar septal capillary congestion, a small amount of inflammatory cell infiltration, or thickened alveolar septa in the vascular lumen and perivascular and interstitial lung.

Fig. 4

Comparison of histopathological examination of each group. A. Histopathological examination of lung in each group. B. Histopathological examination of liver in each group. C. Histopathological examination of kidney in each group. D. Histopathological examination of heart in each group. There was no obvious pathological changes in these main organs.

Fig. 5

Pathological results of injection sites muscle from each group. A. The injection sites muscle from negative control group administrated with saline. B. The injection sites muscle from solvent control group administrated with SARS-CoV-2 inactivated vaccine blank control. C. The injection sites muscle from low-dose group administrated with 100U/dose SARS-CoV-2 inactivated vaccine. D. The injection sites muscle from high-dose group administrated with 150U/dose SARS-CoV-2 inactivated vaccine. There was no significant lesion in each group at the injection sites.