The present invention provides a novel virus-like particle expression vector pTMSCA2C, which is constructed as follows: firstly, using plasmid pTrcHis-MS2 as a starting vector and mutating the base T at position 5 of the gene sequence of MS2 bacteriophage 19mer packaging site on the plasmid pTrcHis-MS2 into C through genetic mutation technologies to obtain a plasmid pTMSC; then, mutating valine which is a amino acid corresponding to the initiation codon on the plasmid pTMSC for encoding the maturase protein of MS2 bacteriophage into methionine to obtain a plasmid pTMSCA; and finally, the gene sequence coding wild type MS2 bacteriophage coat protein, after the removal of the terminator, is linked in series with the gene sequence coding MS2 bacteriophage coat protein comprising histidine-tag which is from a pseudovirus vector pTrcMS, and the gene sequence obtained after linking in series is linked to the plasmid pTMSCA to give pTMSCA2C. When the virus-like particle is prepared by using the expression vector pTMSCA2C of the present invention, the yield and purity of the virus-like particle may be improved while the workload for preparation of virus-like particles may be greatly reduced.

A system for sampling and separating submicron-sized particles to detect the presence of an agent such as viruses in an environmental sample, which includes collecting means for collecting a sample suspected of containing submicron-sized particles from the environment, size separation means receiving the submicron-sized particles from the collecting means for separating the submicron-sized particles based on size into at least one size range, and a microscale particle counter adapted for counting the size separated submicron-sized particles received from the size separation means. The particle counter includes at least one cantilever each corresponding to submicron-sized particles of a particular size range, wherein the cantilever is deflectable from a first to a second position to permit passage of the submicron-sized particle therethrough, and wherein the corresponding deflection of the cantilever generates a count signal.

A chromatography medium comprising porous beads having an inner porous core and an outer porous shell is used for chromatographic separation of enveloped or membranous biological particles from impurities such as contaminant DNA and/or protein. The core is capable of binding molecules via hydrophobic interactions; however, the pore size of the shell does not allow particles having a size of 20 nm and larger to permeate into the bead and interact with the core. The separation is performed at a pH of less than 7.4. The enveloped or membranous biological particles may have been subjected to a prior chromatographic capture step. When used for purification of enveloped virus particles the inventive process was found to yield a remarkably high rate of infectious virus particles.

Provided herein are pseudoviruses expressing a SARS-CoV-2 S glycoprotein. Also provided herein are assays that employ the pseudoviruses to evaluate the immunogenicity of a biological sample against a SARS-CoV-2 virus or variant thereof. Also provided herein are methods of evaluating the immunogenicity of a COVID-19 vaccine using the assays.