{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T15:44:55Z","timestamp":1785426295799,"version":"3.56.0"},"reference-count":32,"publisher":"MDPI AG","issue":"8","license":[{"start":{"date-parts":[[2026,7,28]],"date-time":"2026-07-28T00:00:00Z","timestamp":1785196800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Government Pharmaceutical Organization"},{"DOI":"10.13039\/501100017170","name":"Thailand Science Research and Innovation","doi-asserted-by":"crossref","id":[{"id":"10.13039\/501100017170","id-type":"DOI","asserted-by":"crossref"}]},{"id":[{"id":"https:\/\/ror.org\/000025p04","id-type":"ROR","asserted-by":"publisher"}]},{"DOI":"10.13039\/100031580","name":"National Vaccine Institute","doi-asserted-by":"crossref","id":[{"id":"10.13039\/100031580","id-type":"DOI","asserted-by":"crossref"}]},{"id":[{"id":"https:\/\/ror.org\/02z5tat04","id-type":"ROR","asserted-by":"publisher"}]},{"name":"Program Management Unit for Competitiveness"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Vaccines"],"abstract":"<jats:p>Background\/Objectives: Waning immunity after primary COVID-19 vaccination supports evaluation of additional doses. HXP-GPOVac is an egg-based, inactivated Newcastle disease virus (NDV)-vectored vaccine expressing a prefusion-stabilized SARS-CoV-2 HexaPro spike antigen. We evaluated the safety, tolerability, and immunogenicity of a single additional 10 \u00b5g dose of HXP-GPOVac administered to adults previously primed with two doses of either HXP-GPOVac or BNT162b2. Methods: Study GPO NDV-HXP-S 203 was an open-label phase II extension enrolling adults (18\u201375 years) who previously completed a two-dose primary series in Study 202 with either HXP-GPOVac or BNT162b2 (Pfizer\u2013BioNTech; Comirnaty). All participants received a single additional 10 \u00b5g intramuscular dose of HXP-GPOVac \u2265 6 months after their second primary dose. Solicited local\/systemic adverse events (AEs) were recorded for 7 days, unsolicited AEs through Day 28, and serious AEs (SAEs) and adverse events of special interest (AESIs) throughout follow-up. Neutralizing antibody titers (pseudovirus 50% neutralization titer, NT50) and anti-spike IgG (BAU\/mL) were assessed pre-dose (Day 1) and post-vaccination through 12 months; a predefined subset underwent IFN-\u03b3 and IL-5 ELISpot. SARS-CoV-2 infection during follow-up was assessed using anti-nucleocapsid (anti-N) IgG. Symptomatic COVID-19 was identified through symptom-reported, symptom-triggered RT-PCR testing; sequencing was performed when feasible. Results: All 219 participants received HXP-GPOVac (167 primed with HXP-GPOVac and 52 with BNT162b2). Any solicited local reaction occurred in 22.2% (37\/167) of HXP-GPOVac-primed and 26.9% (14\/52) of BNT162b2-primed participants; any solicited systemic reaction occurred in 10.8% (18\/167) and 13.5% (7\/52), respectively. No vaccine-related unsolicited AEs or AESIs were reported. Three deaths occurred during the 12-month follow-up; one (a sudden cardiac death in an HXP-GPOVac-primed participant) was assessed by the safety medical team as possibly related to vaccination, and two were assessed as not related. Neutralizing antibody GMTs increased from 46.33 at baseline to 1569.04 at Day 15 in HXP-GPOVac-primed participants and from 77.25 to 841.34 in BNT162b2-primed participants; corresponding SCRs were 78.8% and 76.9%. Anti-spike IgG GMCs increased from 48.79 to 1480.14 BAU\/mL and from 194.48 to 1547.88 BAU\/mL, respectively. Responses declined over time but remained above baseline through 12 months. In the cellular immunity subset, post-vaccination IFN-\u03b3 responses increased, with comparatively modest IL-5 responses and no pattern suggestive of Th2 predominance. Conclusions: A single additional dose of HXP-GPOVac administered \u22656 months after primary vaccination with HXP-GPOVac or BNT162b2 was generally well tolerated and elicited robust recall humoral responses, with supportive findings of cellular immunity. Trial registration: Thai Clinical Trials Registry, TCTR20230213001.<\/jats:p>","DOI":"10.3390\/vaccines14080660","type":"journal-article","created":{"date-parts":[[2026,7,28]],"date-time":"2026-07-28T07:55:41Z","timestamp":1785225341000},"page":"660","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Safety and Immunogenicity of an Additional Dose of Thailand Government Pharmaceutical Organization (GPO) Inactivated NDV-HXP-S COVID-19 Vaccine (HXP-GPOVac) Administered After Primary Vaccination with HXP-GPOVac or BNT162b2: An Open-Label Phase II Extension Trial in Thai Adults"],"prefix":"10.3390","volume":"14","author":[{"given":"Prabda","family":"Praphasiri","sequence":"first","affiliation":[{"name":"Influenza and Vaccine Study Center, Nakhon Phanom Provincial Hospital, Nakhon Phanom 48000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Darunee","family":"Ditsungneon","sequence":"additional","affiliation":[{"name":"Faculty of Public Health, Kasetsart University Chalermphrakiat Sakon Nakhon Province Campus, Sakon Nakhon 47000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1055-3656","authenticated-orcid":false,"given":"Anusak","family":"Kerdsin","sequence":"additional","affiliation":[{"name":"Faculty of Public Health, Kasetsart University Chalermphrakiat Sakon Nakhon Province Campus, Sakon Nakhon 47000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5247-0221","authenticated-orcid":false,"given":"Sutthichai","family":"Nakphook","sequence":"additional","affiliation":[{"name":"Influenza and Vaccine Study Center, Nakhon Phanom Provincial Hospital, Nakhon Phanom 48000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0000-1239-0518","authenticated-orcid":false,"given":"Jiraphut","family":"Kittiwatanachod","sequence":"additional","affiliation":[{"name":"Influenza and Vaccine Study Center, Nakhon Phanom Provincial Hospital, Nakhon Phanom 48000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kanlaya","family":"Sornwong","sequence":"additional","affiliation":[{"name":"Influenza and Vaccine Study Center, Nakhon Phanom Provincial Hospital, Nakhon Phanom 48000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Suriya","family":"Naosri","sequence":"additional","affiliation":[{"name":"Influenza and Vaccine Study Center, Nakhon Phanom Provincial Hospital, Nakhon Phanom 48000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sarunpattori","family":"Khunarsa","sequence":"additional","affiliation":[{"name":"Influenza and Vaccine Study Center, Nakhon Phanom Provincial Hospital, Nakhon Phanom 48000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ponthip","family":"Wirachwong","sequence":"additional","affiliation":[{"name":"Government Pharmaceutical Organization (GPO), Bangkok 10400, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Isariya","family":"Techatanawat","sequence":"additional","affiliation":[{"name":"Government Pharmaceutical Organization (GPO), Bangkok 10400, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Piengthong","family":"Narakorn","sequence":"additional","affiliation":[{"name":"Government Pharmaceutical Organization (GPO), Bangkok 10400, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1058-5247","authenticated-orcid":false,"given":"Somchaiya","family":"Surichan","sequence":"additional","affiliation":[{"name":"Government Pharmaceutical Organization (GPO), Bangkok 10400, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jorge","family":"Flores","sequence":"additional","affiliation":[{"name":"PATH, Seattle, WA 98121, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2517-8900","authenticated-orcid":false,"given":"Laina D.","family":"Mercer","sequence":"additional","affiliation":[{"name":"PATH, Seattle, WA 98121, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9801-2012","authenticated-orcid":false,"given":"Christina S.","family":"Polyak","sequence":"additional","affiliation":[{"name":"PATH, Seattle, WA 98121, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1753-7248","authenticated-orcid":false,"given":"Bruce L.","family":"Innis","sequence":"additional","affiliation":[{"name":"PATH, Seattle, WA 98121, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8254-0809","authenticated-orcid":false,"given":"Rama","family":"Raghunandan","sequence":"additional","affiliation":[{"name":"PATH, Seattle, WA 98121, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Chakrarat","family":"Pittayawonganon","sequence":"additional","affiliation":[{"name":"Office of the Permanent Secretary, Ministry of Public Health, Nonthaburi 11000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sopon","family":"Iamsirithaworn","sequence":"additional","affiliation":[{"name":"Bureau of Official Inspection, Office of the Permanent Secretary, Ministry of Public Health, Nonthaburi 11000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Supakit","family":"Sirilak","sequence":"additional","affiliation":[{"name":"Health Systems Research Institute, Nonthaburi 11000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8728-5405","authenticated-orcid":false,"given":"Kriengkrai","family":"Prasert","sequence":"additional","affiliation":[{"name":"Influenza and Vaccine Study Center, Nakhon Phanom Provincial Hospital, Nakhon Phanom 48000, Thailand"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,7,28]]},"reference":[{"key":"ref_1","unstructured":"World Health Organization (2025). 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