Table 1.

Recommended immunological surrogate endpoints for licensed viral vaccines.

Vaccine name	Vaccine type	Route of virus transmission	Recommended clinical surrogate endpoint for vaccine	Ref
Influenza vaccines	Inactivated vaccines	Respiratory tract	1. Significant increase in seroconversion factor or hemagglutination inhibition (HI) antibody titer of > 40%; 2. Increase in GMT of > 2.5; 3. Proportion of subjects with HI titer ≥ 1:40 or single radial hemolysis (SRH) area > 25 mm² > 70%	(11)
	Split virus vaccines
	Subunit vaccines
Measles vaccines	Live attenuated vaccines	Respiratory tract	Hemagglutinin (H)- and fusion protein (F)-specific neutralizing antibody titer > 120 mIU/mL in the plaque reduction neutralization test (PRNT)	(12)
Japanese encephalitis vaccines	Live attenuated vaccines	Mosquito vectors	Baseline negative: PRNT50 ≥ 1:10	(13)
			Baseline positive: 4-fold increase in PRNT50
Rabies vaccines	Inactivated and live attenuated vaccines	Animals	The WHO states that vaccines with a potency of 2.5 IU/dose can induce adequate immunogenicity and provide protective effects with the generation of antibody concentrations of > 0.5 IU	(14)
Polio vaccines	Inactivated and live attenuated vaccines	Gastrointestinal tract	Oral live attenuated vaccine: neutralizing antibody titer of 1:4–1:8 or 4-fold increase in antibody titer; inactivated vaccine: neutralizing antibody titer of 1:8 or 4-fold increase in antibody titer	(15, 16)
Hepatitis A vaccines	Inactivated and live attenuated vaccines	Gastrointestinal tract	Anti-hepatitis A virus (HAV) IgG ≥ 20 mIU/mL	(17)
Enterovirus 71 (EV71) vaccines	Inactivated vaccines	Gastrointestinal tract	Neutralizing antibody titer of 1:32	(18, 19)
Varicella vaccines	Live attenuated vaccines	Contact	Titer of antibodies to the varicella-zoster virus (VZV) glycoprotein measured by ELISA ≥ 5 gpELISA units	(20)
Hepatitis B vaccines	Recombinant vaccines	Blood	Percentage of subjects with hepatitis B surface antibody (anti-HB) titer ≥10 mIU/mL	(21)