With more people getting their COVID-19 vaccines, a larger discourse on vaccination is occurring. Many questions are being presented. How are vaccines made? What is the safety of a vaccine? How is the efficacy of a vaccine determined? How small are vaccine particles? This blog discusses some history of vaccine development and several ways Particle Technology Labs supports the characterization of today’s vaccines.
History of Vaccines
Smallpox was an infectious disease that had a mortality rate of ≤30%. Variolation was developed as a process to inoculate individuals from smallpox (Variola major/minor). Variolation was achieved through the use of powdered smallpox scabs or fluid from pustules being introduced to a patient through scratches on their skin or inhalation through the nose. Variolation was successful in reducing the mortality rate of smallpox to <1% in the variolated patients. However, the variolated patients were able to transmit the virus causing secondary outbreaks.
Towards the end of the 18th century, Edward Jenner observed that milkmaids who had been exposed to a pox-like disease (cowpox, Variolae vaccinae) were immune to smallpox. As an experiment, Dr. Jenner inoculated 9-year-old James Phipps, utilizing pustules from milkmaid Sarah Nelmes’ cowpox sore. After a period of time, the patient was exposed to the smallpox virus several times with no adverse symptoms after each exposure. Dr. Jenner’s findings would lead to the adoption of vaccination over variolation.
Since then, many forms of active components have been employed, from the use of inactivated or attenuated viruses to the modern approach of using ribonucleic acids (RNA). A vaccine formula today also typically contains several other ingredients, which may enhance the immune response or help ensure its stability.
The Impact of Particle Size on Antigen Uptake
A vaccine’s safety and efficacy can depend on many factors. At Particle Technology Labs, we have characterized various materials utilized in vaccines. One specific characterization Particle Technology Labs conducts is particle size, which relates to antigen uptake.
To create an immunological memory, our immune system utilizes key antigens associated with a specific pathogen in order to recognize the pathogen as a foreign body. Exposure to these antigens through vaccination is intended to trigger a person’s immune responses when exposed to similar antigens in the future. For the success of inoculation, proper antigen uptake must occur. Many vehicles are utilized for antigen uptake, such as liposomes, oil in water emulsions, water in oil emulsions, virosomes, etc. Particle Technology Labs regularly tests these vaccine components for particle size and other important characteristics.
One critical factor in acquired immune response is transporting the antigen to the lymphoid organs. Manolova V. et al. evaluated the size dependence of particles migrating to the lymph node in a study published in 2008. In this study, particles ≤200 nm were shown to have rapid uptake in the lymph node, while an optimal range of 10 to 100 nm was noted from previous studies. Larger 500 nm and 1000 nm particles were shown to arrive at the lymph node but required significantly more time to reach the destination. A study by Fifis T. et al. suggested a target size of 40 nm for optimal immune response. Oussoren C. et al. concluded that size was the most critical factor in the lymphatic uptake of liposomes.
Dynamic Light Scattering (DLS) and Nano Tracking Analysis (NTA) Excellent for Vaccine Material Particle Size Testing
For suspended material, Particle Technology Labs utilizes the theory posed by the Stokes-Einstein equation. This theory equates the diffusion of suspended material freely moving in Brownian motion to particle size. In simple terms, small particles move rapidly, while larger particles move slowly in a suspension. Particle Technology Labs utilizes two techniques that apply this theory, Dynamic Light Scattering (DLS) and Nano Tracking Analysis (NTA).
DLS evaluates the suspended material concurrently, generating an average intensity-weighted particle size. For studies conducted at Particle Technology Labs, this technique has been utilized as a Quality Control (QC) tool when evaluating consistency between batches. DLS is a great QC tool as it has a high sensitivity to small amounts of agglomeration/aggregation. Since size is critical to antigen uptake, agglomeration/aggregation can have a significant impact on the efficacy of a vaccine.
NTA applies the same theory as DLS but conducts it on a per-particle basis. This means the technique is highly resolute, providing number-weighted results that can be broken down by individual population sizes. With the high resolution of this technique, it can handle sample concentrations significantly more dilute than DLS. Particle Technology Labs has conducted studies utilizing this technique to aid in the development and optimization of vaccine materials. It has also been utilized as a QC tool when particle concentration is low.
The Importance of Data Weighting in Particle Size Determination
With any particle size technique, the results are presented relative to a specific weighting. The three most common size weightings are number, volume, and intensity. Number-weighted results present the data where each particle has the same representation in the distribution. A 100 nm particle has the same impact on the distribution as a 1 nm particle.
Volume-weighted results are skewed towards the larger particles. This is because a larger particle has significantly more volume than a smaller particle. Using our previous example, a 100 nm particle would have the same impact as approximately eight thousand 1 nm particles on a volume-weighted basis. The skew of the results is beneficial as this weighting is a good approximation of where most of the sample mass resides within the size distribution. This is due to larger particles being attributed to a larger portion of the material’s mass.
Specific to the DLS technique is the intensity-weighted distribution. This weighting places the emphasis on the intensity of light scattered by the particle. A single 100 nm particle has the same impact on the distribution as 1012 1 nm particles. The significance of the intensity of light scatter is what gives DLS its sensitivity to the presence of small amounts of agglomeration/aggregation.
How Can Particle Technology Labs Help You With Nanomaterial Characterization?
Particle Technology Labs has the capability to determine the particle size of nano-suspended materials. Particle Technology Labs has conducted many studies optimizing methodology to be both accurate and precise. Malvern Panalytical’s Zetasizer Nano (DLS), Anton Paar’s Litesizer (DLS), and Malvern Panalytical’s Nanosight NS300 (NTA) are the technologies utilized at Particle Technology Labs for nanomaterial characterization. These instruments can be utilized in the optimization of a vaccine targeting the lymphatic system based on the size of the vaccine vehicle.