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Assessment of serological responses following vaccination campaigns with type 2 novel oral polio vaccine: a population-based study in Tajikistan in 2021

BACKGROUND

Novel oral poliovirus vaccine type 2 (nOPV2) was used to control an outbreak of type 2 circulating vaccine derived poliovirus (cVDPV2) in Tajikistan, in 2021. We measured seroconversion and seroprevalence of type 2 polio antibodies in children who were reported to have received two doses of nOPV2 in outbreak response campaigns.

METHODS

In this community serosurvey, children born after Jan 1, 2016 were enrolled from seven districts in Tajikistan. Dried blood spot cards were collected before nOPV2 campaigns and after the first and second rounds of the campaigns and were sent to the Centers for Disease Control and Prevention (Atlanta, GA, USA) for microneutralisation assay to determine presence of polio antibodies. The primary endpoint was to assess change in seroprevalence and seroconversion against poliovirus serotype 2 after one and two doses of nOPV2.

FINDINGS

228 (97%) of 236 enrolled children were included in the analysis. The type 2 antibody seroprevalence was 26% (53/204; 95% CI 20 to 33) before nOPV2, 77% (161/210; 70 to 82) after one dose of nOPV2, and 83% (174/209; 77 to 88) after two doses of nOPV2. The increase in seroprevalence was statistically significant between baseline and after one nOPV2 dose (51 percentage points [42 to 59], p<0·0001), but not between the first and second doses (6 percentage points [-2 to 15], p=0·12). Seroconversion from the first nOPV2 dose, 67% (89/132; 59 to 75), was significantly greater than that from the second nOPV2 dose, 44% (20/45; 30 to 60; χ² p=0·010). Total seroconversion after two nOPV2 doses was 77% (101/132; 68 to 83).

INTERPRETATION

Our study demonstrated strong immune responses following nOPV2 outbreak response campaigns in Tajikistan. Our results support previous clinical trial data on the generation of poliovirus type 2 immunity by nOPV2 and provide evidence that nOPV2 can be appropriate for the cVDPV2 outbreak response. The licensure and WHO prequalification of nOPV2 should be accelerated to facilitate wider use of the vaccine.

FUNDING

World Health Organization, Centers for Disease Control and Prevention, and Rotary International.

Controlled Human Infection Models To Accelerate Vaccine Development

The timelines for developing vaccines against infectious diseases are lengthy, and often vaccines that reach the stage of large phase 3 field trials fail to provide the desired level of protective efficacy. The application of controlled human challenge models of infection and disease at the appropriate stages of development could accelerate development of candidate vaccines and, in fact, has done so successfully in some limited cases. Human challenge models could potentially be used to gather critical information on pathogenesis, inform strain selection for vaccines, explore cross-protective immunity, identify immune correlates of protection and mechanisms of protection induced by infection or evoked by candidate vaccines, guide decisions on appropriate trial endpoints, and evaluate vaccine efficacy. We prepared this report to motivate fellow scientists to exploit the potential capacity of controlled human challenge experiments to advance vaccine development. In this review, we considered available challenge models for 17 infectious diseases in the context of the public health importance of each disease, the diversity and pathogenesis of the causative organisms, the vaccine candidates under development, and each model's capacity to evaluate them and identify correlates of protective immunity. Our broad assessment indicated that human challenge models have not yet reached their full potential to support the development of vaccines against infectious diseases. On the basis of our review, however, we believe that describing an ideal challenge model is possible, as is further developing existing and future challenge models.

Enabling accelerated vaccine roll-out for Public Health Emergencies of International Concern (PHEICs): Novel Oral Polio Vaccine type 2 (nOPV2) experience

To address the evolving risk of circulating vaccine-derived poliovirus type 2 (cVDPV2), Global Polio Eradication Initiative (GPEI) partners are working closely with countries to deploy an additional innovative tool for outbreak response - novel oral polio vaccine type 2 (nOPV2). The World Health Organization's (WHO) Prequalification program issued an Emergency Use Listing (EUL) recommendation for nOPV2 on 13 November 2020. The WHO's EUL procedure was created to assess and list unlicensed vaccines, therapeutics and diagnostics to enable their use in response to a Public Health Emergency of International Concern (PHEIC). nOPV2 was the first vaccine to receive an EUL, paving the way for other emergency vaccines. In this report, we summarise the pathway for nOPV2 roll-out under EUL.

Viewpoint of a WHO Advisory Group Tasked to Consider Establishing a Closely-monitored Challenge Model of Coronavirus Disease 2019 (COVID-19) in Healthy Volunteers

WHO convened an Advisory Group (AG) to consider the feasibility, potential value, and limitations of establishing a closely-monitored challenge model of experimental severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and coronavirus disease 2019 (COVID-19) in healthy adult volunteers. The AG included experts in design, establishment, and performance of challenges. This report summarizes issues that render a COVID-19 model daunting to establish (the potential of SARS-CoV-2 to cause severe/fatal illness, its high transmissibility, and lack of a "rescue treatment" to prevent progression from mild/moderate to severe clinical illness) and it proffers prudent strategies for stepwise model development, challenge virus selection, guidelines for manufacturing challenge doses, and ways to contain SARS-CoV-2 and prevent transmission to household/community contacts. A COVID-19 model could demonstrate protection against virus shedding and/or illness induced by prior SARS-CoV-2 challenge or vaccination. A limitation of the model is that vaccine efficacy in experimentally challenged healthy young adults cannot per se be extrapolated to predict efficacy in elderly/high-risk adults.

Reduced mucosal immunity to poliovirus after cessation of trivalent oral polio vaccine

The switch from using only trivalent oral polio vaccine (tOPV) to sequential schedules combining inactivated poliovirus vaccine (IPV) and bivalent oral polio vaccine (bOPV) for polio vaccination will cause changes to mucosal immunity against polio in infants, which plays an important role in preventing the poliovirus spread. Here, we analyzed mucosal immunity against poliovirus in the intestine during different sequential vaccination schedules. We conducted clinical trials in Guangxi Province, China on 1,200 2-month-old infants who were randomly assigned to one of three vaccination schedule groups: IPV-bOPV-bOPV, IPV-IPV-tOPV, and IPV-IPV-bOPV, with vaccine doses administered at 8, 12, and 16 weeks of age. Stool samples were collected from 10% of participants in each group before administration of the second vaccine doses and at 1, 2, and 4 weeks after the administrations of the second and third vaccine doses. Immunoglobulin A (IgA) in the stool samples was measured to analyze the mucosal immune response in the intestine. Because of the absence of poliovirus type 2 in bOPV, the vaccination schedule of IPV-IPV-bOPV did not sufficiently raise intestinal mucosal immunity against poliovirus type 2, although some cross-immunity was seen. The level of intestinal mucosal immunity was related to shedding status; shedders could produce intestinal mucosa IgA more quickly. The intestinal mucosal immunity level was not related to serum neutralizing antibody level. In the combined sequential vaccination schedule of IPV and bOPV, the risk of circulating vaccine-derived poliovirus type 2 (cVDPV2) may be increased owing to insufficient intestinal mucosal immunity against poliovirus type 2.

Intestinal antibody responses to two novel live attenuated type 2 oral poliovirus vaccines in healthy adults in Belgium

In a blinded phase 1 trial (EudraCT 2017-0000908-21; NCT03430349) in Belgium, healthy adults (aged 18–50 years) previously immunized exclusively with inactivated poliovirus vaccine were administered a single dose of 1 of 2 novel type 2 oral poliovirus vaccines (nOPV2-c1: S2/cre5/S15domV/rec1/hifi3 (n = 15); nOPV2-c2: S2/S15domV/CpG40 (n = 15)) and isolated for 28 days in a purpose-built containment facility. Using stool samples collected near days 0, 14, 21, and 28, we evaluated intestinal neutralization and immunoglobulin A responses to the nOPV2s and found that nOPV2-c1 and nOPV2-c2 induced detectable poliovirus type 2–specific intestinal neutralizing responses in 40.0% and 46.7% of participants, respectively.