Single administration vaccines: delivery challenges, in vivo performance, and translational considerations

Figures & data

Figure 1. Coverage rates of DTP vaccine at 5-year intervals in different world regions (1995–2019) generated from data extracted from UNICEF database. DTP1, Diphtheria/Tetanus/Pertussis dose 1; DTP3, Diphtheria/Tetanus/Pertussis dose 3.Note: Data obtained from https://data.unicef.org/resources/dataset/immunization/

Table 1. List of potential vaccine candidates for SAV formulation.

	Vaccine		Type	Recommended schedule
Routine immunization	Hepatitis B vaccine		Recombinant Subunit	Primary series of 3 doses: - As soon as possible after birth - Gap of 4 weeks between doses Unvaccinated individuals vaccinated with a 0, 1, 6-month schedule
	Diphtheria (DTP)		Toxoid	Primary series of 3 doses: - Earliest start: 6 weeks of age - Gap of 4 weeks between doses
	Tetanus (DTP)
	Whooping cough (DTP)
	Haemophilus influenza type b (Hib)		Subunit/conjugate	Primary series of 3 dose - Earliest start: 6 weeks of age - Gap of 4 weeks between doses
	Polio (IPV)		Inactivated	Primary series of 3 doses: - Earliest start: 8 weeks of age - Gap of 4 weeks between doses Course of 3 doses given to unimmunized adults
	Human Papillomavirus (HPV)		Recombinant Subunit	Prime dose given at the age 12–13 followed by a booster dose after at least 6 months. A three-dose schedule (0, 1–2, 6 months) used for vaccinations initiated ≥15 years of age or in immunocompromised people.
Conditional*	Varicella Zoster vaccine		Live Attenuated	Second dose 4 to 8 weeks after the prime
	Japanese encephalitis vaccine		Inactivated	Second dose 28 days after the prime, but people aged 18–64 can have the second dose 7 days after prime.
	Rabies vaccine		Inactivated	Three doses over a period of 28 days
	Dengue vaccine (CYD-TDV)		Live attenuated	Prime dose after 9 years of age followed by a second dose 6 months later (plus booster dose at 12 months)
Pandemic	COVID-19 vaccine	Spikevax (Moderna)	mRNA	Second dose 28 days after the prime.
		Comirnaty (Pfizer/BioNTech)	mRNA	Second dose at least 21 days after the prime
		Vaxzevria (Oxford/AstraZeneca)	Adenovirus-vectored DNA	Second dose 4 to 12 weeks after the prime
		Jcovden (Janssen)	Adenovirus-vectored DNA	Booster dose at least 2 months after the prime
		Nuvaxovid (Novavax)	Recombinant Subunit	Second dose 3 weeks after the prime
		COVID-19 Vaccine Valneva (Valneva)	Inactivated	Second dose 4 weeks after the prime

Note: *Conditional depending on risk factors e.g. location, exposure.

Figure 2. (a) Theoretical Triphasic release pattern from PLGA particles: initial antigen release from the surface of the microparticles through diffusion resulting in a burst phase (b), diffusion-dependent constant release phase (c), second rapid release phase caused by polymer erosion (R); (b) Corresponding theoretical antigen concentration in blood.

Figure 3. Factors influencing antigen release from PLGA microspheres.

Table 2. Notable Single administration vaccine approaches.

Antigen	Excipients		Microparticle fabrication method	Release profile	Immune response	Pros (+)/Cons (−)	Reference
	Stabilizers	Release technology
Tetanus toxoid	Trehalose, albumin, poloxamer 188, stabilizer no. 1 (commercial mixture of saccharides, amino acids, and electrolytes), stabilizer no. 176 (commercial mixture of saccharides, amino acids, electrolytes, and a protein)	PLGA 50:50, PLA	Spray-drying, Solvent evaporation, Coacervation	N/A	-All formulations generated antibody titers higher than the protection limit maintained for 12 weeks. -Tetanus toxin neutralization capacity Control -> 20 IU/ml SAV -> 5–15 IU/ml	(+) Various combinations of stabilizers, release technologies, and microparticle fabrication methods were compared	[57]
IPV serotypes 1,2,3	- Sucrose, sorbitol, trehalose, maltodextrin in combination with monosodium glutamate and magnesium Chloride were effective on maintaining stability of IPV on incubation at 37°C, sonication and vacuum drying. - Gelatin: did not show improvement in stability during the incubation but prevented loss during drying and sonication	PLGA 50:50 with Eudragit E	Double emulsion	EPO allowed tuning of release kinetics: − 7.5% EPO 2nd peak → 11 days − 5% EPO 2nd peak → 14–18 days − 3% EPO 2nd peak → 25 days Addition of arginine: − 2nd peak → 32 days.	The formulation: (8% maltodextrin, 6.8% MSG, 6.8% MgCl2, 3% EPO) displayed a non-inferior response to the control as measured by ELISA.	(+) Excipients tested in different combinations and concentrations. (−) Stabilizer’s screening (incubation at 37°C, sonication and drying process) to evaluate excipients stabilizing performance not done in the presence of PLGA	[58]
IPV serotypes 1,2,3	- Cationic polymer excipients Eudragit E, poly (L-lysine), and branched poly-ethylenimine were used to increase stability and prevent protein aggregation caused by low pH. - Small-molecule excipients for IPV protection (17% MSG, 20% maltodextrin, and 17% MgCl2)	PLGA 50:50 with EPO or PLL + bPEI	Double emulsion	Pulsatile release was observed as the three IPV serotypes were released in two distinct bursts. − 2nd peak → 20–30 days	Non-inferior immune response to the control for both SAV formulations.	(+) Use of cationic polymers to modulate and achieve pulsatile release	[59]
IPV and Ovalbumin	Sucrose, monosodium glutamate, and magnesium chloride.	PLGA: − 50:50 ester end group − 50:50 acid end group − 85:15 acid end group	StampEd Assembly of polymer Layers (SEAL)	Fluorescently labeled dextran: In vitro: no burst release, day 10 (1st peak), day 15 (2nd peak) day 34 (3rd peak) In vivo: no burst release, day 9 (1st peak), day 20 (2nd peak) day 41 (3rd peak)	Peak titers were higher than the control (two-dose boluses)	(+) Pulsatile release: delayed bursts without prior leakage (i.e. delayed sharp bursts on a specific day) (−) Non-traditional manufacturing process platform likely to be related with cost-effectiveness and scalability	[60]
HPV16 L1 Capsomere (model antigen)	-Trehalose (protect antigen during drying) -Hydroxyethyl starch (raises glass transition temperature increasing stability and preventing agglomeration)	-Trimethyl-aluminium	- Spray-drying (to stabilize the antigen) - Atomic layer deposition	HPV16 L1 labeled with IR Dye 800CW -In vivo: No coats ≈ 1–3 weeks 100 coats ≈ 4 weeks 250–500 coats ≈ 14 weeks	-Antibody titers similar/better to the control (prime + booster dose)	(+) Alumina layers acted both as adjuvants and as release technology (−) Uncontrolled fracture of particles in lab-scale atomic layer deposition reactor	[61]

Note: IPV, inactivated polio vaccine; SAV, single administration vaccine; EPO, Eudragit E; MSG, monosodium glutamate; MgCl₂, magnesium chloride; ELISA, enzyme-linked immunosorbent assay; PLL, poly (L-lysine); bPEI, poly-ethylenimine; PLGA, Poly lactic glycolic acid; PLA, poly lactic acid.

Table 3. Animal studies of hepatitis B single administration vaccines (SAVs).

SAV formulation	Immune response	Control	Duration	Model (route)	Ref.
HBsAg(7.5 μg) PLGA 50/50-COOH microspheres	Inferior results after the sixth week	2.5 μg HBsAg aluminium-vaccine at 0, 1, 2 months	4 months	Mouse (SC)	[68]
HBsAg(7.5 μg) PLGA 50/50 microspheres	Comparable serum antibody titres to control
HBsAg(7.5 μg) PLGA 75/25 microspheres
HBsAg(7.5 μg) mixture of PLGA 50/50, PLGA 75/25, PLGA 50/50 - COOH microspheres
HBsAg(30 μg) PLG505, PLG858 (<10 μm) and PLA208 (>10 μm) microspheres	Antibody response comparable to control	10 μg HBsAg aluminium-vaccine at 0, 1, 6 months	12 months	Mouse (IM)	[69]
HBsAg(30 μg) PLG505 (<10 μm) and PLG858, PLA208 (>10 μm) microspheres
HBsAg(20 μg) PLGA 50/50 with trehalose and Mg(OH)2 microspheres	Immune response comparable to the control	10 μg HBsAg aluminium-vaccine at 0 and 1 months	3 months	Guinea pig (SC)	[39]
HBsAg(20 μg) PLGA 50/50 with Mg(OH)2 only	Significantly lower immune response than the control
HBsAg(20 μg) PLGA 50/50 with trehalose only
HBsAg(10 μg) Alginate–chitosan–PLGA (50/50, 70/30) composite microspheres	Antibody levels comparable to control	5 μg HBsAg aluminium-vaccine at 0 and 1 months	3 months	Mouse (SC)	[70]
HBsAg(10μg) PLGA (50/50, 70/30) microspheres	Significantly lower at all sampling times
HBsAg(20 μg) adsorbed microspheres: PLGA 50/50	Comparable immune response	10 μg HBsAg aluminium-vaccine at 0 and 1 months	42 days	Mouse (SC)	[72]
HBsAg(20 μg) adsorbed microspheres: PLGA 75/25
HBsAg(1 μg) loaded PLA nanospheres	Inferior antibody titers	Total 1 μg HBsAg aluminium-vaccine at 0, 1, 6 months	6 months	Rat (IM)	[71]
HBsAg(1 μg) loaded PLA nanospheres with alum
HBsAg(1 μg) loaded PLA microspheres
HBsAg(1 μg) loaded PLA microspheres with alum	Comparable antibody titres to the control

Note: SC, subcutaneous; IM, intramuscular; HBsAg, hepatitis surface antigen; PLGA, poly lactic glycolic acid; PLA, poly lactic acid.

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