1. Study design and subjects
This was a descriptive study performed at Tokyo Medical University Hospital in Japan between May 26, 2011 and August 31, 2016 (UMIN000013551). The study was approved by the ethics committees of the Tokyo Medical University and National Institute of Infectious Diseases, and was conducted following the ethical principles laid out in the Declaration of Helsinki. All subjects provided written informed consent before participating in the study.
Eligible participants were aged 20 years or older. Their childhood polio vaccination history was obtained from immunization records.
Exclusion criteria were pregnancy or breastfeeding, history of poliomyelitis or polio infection, previous IPV vaccination, past history of receiving more than two doses of OPV, known or suspected congenital or acquired immunodeficiency, receipt of immunosuppressive therapy, bleeding disorders, and systemic illness.
During the study, each participant visited Tokyo Medical University Hospital three times. At the first visit, the informed consent form was signed and each participant received cIPV after their eligibility was confirmed. The second and third visits were made between 4 and 6 weeks following the previous visit (Fig. 1).
2. Vaccines
The study vaccine was standalone cIPV (Imovax Polio®, Sanofi Pasteur Inc., Swiftwater, PA, USA. and Imovax Polio® subcutaneous, Sanofi K.K., Tokyo, Japan). In 2011–2012, Imovax Polio® was not approved in Japan, so we imported these vaccines privately. In 2012, Imovax Polio® subcutaneous was approved in Japan, so this vaccine was used in this study. Each subject received ‘Imovax Polio®’ intramuscularly in the deltoid region or ‘Imovax Polio® subcutaneous’ subcutaneously in the triceps region.
Each vaccine was a trivalent IPV provided in a pre-filled syringe with a needle representing a single dose (0.5 mL). The vaccine contained three types of inactivated poliovirus D-antigens: 40 DU Type 1 (Mahoney strain), 8 DU Type 2 (MEF-l strain), and 32 DU Type 3 (Saukett strain). In addition, 2-phenoxyethanol and formaldehyde were included as preservatives. The vaccines were stored at 2–8 °C.
3. Immunogenicity evaluation and serological analysis
Blood samples (6 mL) were collected in dry sterile capped plastic tubes for assessment of neutralizing antibody titers against polioviruses prior to and 4–6 weeks post-vaccination. Blood was allowed to clot and serum was separated by centrifuging at 3000 rpm for 10 min. The serum samples were stored at ≤ − 20 °C.
Immunogenicity was assessed by measuring serum neutralization titers against type 1, 2, and 3 polioviruses in HEp-2 cells using a microneutralization assay. Neutralizing antibody titers were measured at the National Institute of Infectious Diseases (Tokyo, Japan). Viruses included Sabin strains (type 1, 2, and 3), virulent poliovirus strains (type1: Mahoney strain; type 2: MEF-1 strain; and type 3: Saukett strain) and type 2 VDPVs derived from sporadic cases of acute flaccid paralysis in Vietnam in 2012 (SV3128 and SV3130) and from cases from an outbreak in Nigeria in 2005 (11196 and 11198).
The titer of neutralizing antibody required for protection against Sabin strains, virulent poliovirus strains and type 2 VDPVs was assumed to be 1:8 (1/dil) or higher. Neutralizing antibody titers below the threshold of 1:4 were assigned a value of 1:4, and those above the threshold of 1:1024 were assigned a value of 1:1024.
4. Safety
Subjects were observed for 30 min following immunization to assess the occurrence of any immediate adverse events (AEs). Subjects were provided with diary cards to record solicited injection site and systemic reactions as well as other unsolicited AEs. Solicited injection site reactions (pain, erythema, and swelling) and solicited systemic reactions (headache, malaise, myalgia, and fever) were recorded daily for 7 days post-vaccination along with any action taken to manage these AEs. Body temperature was measured daily for 7 days post-immunization. Unsolicited AEs were recorded for 28 days post-immunization.
The intensity of solicited and unsolicited AEs was graded using a three-point scale of increasing severity (grades 1–3). Pain at the injection site was graded as follows: grade 1, no interference with daily activity; grade 2, some interference with daily activity; and grade 3, prevents daily activity. Injection site erythema and swelling were classified as follows: grade 1, ≥ 2.5 and ≤ 5 cm; grade 2, ≥ 5.1 and < 10 cm; and grade 3, ≥ 10 cm. Systemic reactions (headache, malaise, and myalgia) were graded as follows: grade 1, no interference with daily activity; grade 2, some interference with daily activity; and grade 3, prevents daily activity. Fever was defined as axillary temperature ≥ 37.5 °C and was graded as follows: grade 1, ≥ 37.5 °C and ≤ 38.4 °C; grade 2, ≥ 38.5 °C and ≤ 38.9 °C; and grade 3, ≥ 39.0 °C. Serious AEs were recorded throughout the study.
5. Statistical analysis
All statistical analyses were performed using IBM SPSS statistics, version 24 (IBM Corp., Armonk, NY, USA). For immunogenicity assessment, seropositivity rates, geometric mean titers (GMTs) and 95% confidence intervals (CIs) were calculated at Visit 1 (pre-vaccination) and at Visits 2 and 3 post-vaccination with cIPV. Seropositivity was defined as the percentage of subjects with neutralizing antibody titers of ≥ 1:8. For safety evaluation, the numbers and percentage of subjects who experienced at least one AE were calculated.