Congenital Mpox Syndrome (Clade I) in Stillborn Fetus after Placental Infection and Intrauterine Transmission, Democratic Republic of the Congo, 2008.

We report the autopsy pathology findings of a 21-week stillborn fetus with congenital mpox syndrome that occurred in the Democratic Republic of the Congo in 2008. The fetus acquired mpox from the mother after intrauterine transplacental monkeypox virus transmission. We confirmed monkeypox virus infection in the mother, fetus, and placenta by using a monkeypox virus-specific quantitative PCR. Subtyping of the virus was not performed, but the mother and fetus were almost certainly infected with the clade I variant that was endemic in the Democratic Republic of the Congo at the time. Risk for intrauterine infection appears to differ between virus clades, but clinicians should be aware of potential for intrauterine monkeypox virus transmission among pregnant persons during ongoing and future mpox outbreaks.

Mpox (Monkeypox) in Pregnancy: Viral Clade Differences and Their Associations with Varying Obstetrical and Fetal Outcomes.

In African countries where mpox (monkeypox) is endemic, infection is caused by two genetically related clades-Clade I (formerly Congo Basin), and Clade IIa (formerly West Africa), both of which are potentially life-threatening infections. Prior to the 2022-2023 global outbreak, mpox infections among pregnant women caused by Clade I were reported to have a 75% perinatal case fatality rate in the Democratic Republic of Congo, including the only documented case of placental infection and stillbirth from the Congenital Mpox Syndrome, and the Clade IIa mpox infection was associated with stillbirths in Nigeria. The 2022-2023 global mpox outbreak, caused by a genetically distinct strain, Clade IIb, has focused attention on the effects of mpox on pregnant women and fetal outcomes. There have been at least 58 cases of mpox infection occurring in pregnant women during the 2022-2023 outbreak. No confirmed cases of adverse perinatal outcome, including stillbirth, have been reported. The absence of perinatal morbidity and mortality from Clade IIb corresponds to the overall case fatality rate among non-pregnant women of <0.1%, as this clade has been demonstrated to produce a less-severe disease than the mpox Clade I or IIa variants. Thus, there are apparently important differences between mpox clades affecting pregnant women and perinatal outcomes.

Clinical characterization and placental pathology of mpox infection in hospitalized patients in the Democratic Republic of the Congo.

We describe the results of a prospective observational study of the clinical natural history of human monkeypox (mpox) virus (MPXV) infections at the remote L'Hopital General de Reference de Kole (Kole hospital), the rainforest of the Congo River basin of the Democratic Republic of the Congo (DRC) from March 2007 until August 2011. The research was conducted jointly by the Institute National de Recherche Biomedical (INRB) and the US Army Medical Research Institute of Infectious Diseases (USAMRIID). The Kole hospital was one of the two previous WHO Mpox study sites (1981-1986). The hospital is staffed by a Spanish Order of Catholic Nuns from La Congregation Des Soeurs Missionnaires Du Christ Jesus including two Spanish physicians, who were members of the Order as well, were part of the WHO study on human mpox. Of 244 patients admitted with a clinical diagnosis of MPXV infection, 216 were positive in both the Pan-Orthopox and MPXV specific PCR. The cardinal observations of these 216 patients are summarized in this report. There were three deaths (3/216) among these hospitalized patients; fetal death occurred in 3 of 4 patients who were pregnant at admission, with the placenta of one fetus demonstrating prominent MPXV infection of the chorionic villi. The most common complaints were rash (96.8%), malaise (85.2%), sore throat (78.2%), and lymphadenopathy/adenopathy (57.4%). The most common physical exam findings were mpox rash (99.5%) and lymphadenopathy (98.6%). The single patient without the classic mpox rash had been previously vaccinated against smallpox. Age group of less than 5 years had the highest lesion count. Primary household cases tended to have higher lesion counts than secondary or later same household cases. Of the 216 patients, 200 were tested for IgM & IgG antibodies (Abs) to Orthopoxviruses. All 200 patients had anti-orthopoxvirus IgG Abs; whereas 189/200 were positive for IgM. Patients with hypoalbuminemia had a high risk of severe disease. Patients with fatal disease had higher maximum geometric mean values than survivors for the following variables, respectively: viral DNA in blood (DNAemia); maximum lesion count; day of admission mean AST and ALT.

Mpox Virus in Pregnancy, the Placenta, and Newborn.

CONTEXT.­: Before its eradication, the smallpox virus was a significant cause of poor obstetric outcomes, including maternal and fetal morbidity and mortality. The mpox (monkeypox) virus is now the most pathogenic member of the Orthopoxvirus genus infecting humans. The 2022 global mpox outbreak has focused attention on its potential effects during pregnancy. OBJECTIVE.­: To understand the comparative effects of different poxvirus infections on pregnancy, including mpox virus, variola virus, vaccinia virus, and cowpox virus. The impact on the pregnant individual, fetus, and placenta will be examined, with particular attention to the occurrence of intrauterine vertical transmission and congenital infection. DATA SOURCES.­: The data are obtained from the authors' cases and from various published sources, including early historical information and contemporary publications. CONCLUSIONS.­: Smallpox caused maternal and perinatal death, with numerous cases reported of intrauterine transmission. In endemic African countries, mpox has also affected pregnant individuals, with up to a 75% perinatal case fatality rate. Since the start of the 2022 mpox outbreak, increasing numbers of pregnant women have been infected with the virus. A detailed description is given of the congenital mpox syndrome in a stillborn fetus, resulting from maternal-fetal transmission and placental infection, and the potential mechanisms of intrauterine infection are discussed. Other poxviruses, notably vaccinia virus and, in 1 case, cowpox virus, can also cause perinatal infection. Based on the historical evidence of poxvirus infections, mpox remains a threat to the pregnant population, and it can be expected that additional cases will occur in the future.

Design and Optimization of a Monkeypox virus Specific Serological Assay.

Monkeypox virus (MPXV), a member of the Orthopoxvirus (OPXV) genus, is a zoonotic virus, endemic to central and western Africa that can cause smallpox-like symptoms in humans with fatal outcomes in up to 15% of patients. The incidence of MPXV infections in the Democratic Republic of the Congo, where the majority of cases have occurred historically, has been estimated to have increased as much as 20-fold since the end of smallpox vaccination in 1980. Considering the risk global travel carries for future disease outbreaks, accurate epidemiological surveillance of MPXV is warranted as demonstrated by the recent Mpox outbreak, where the majority of cases were occurring in non-endemic areas. Serological differentiation between childhood vaccination and recent infection with MPXV or other OPXVs is difficult due to the high level of conservation within OPXV proteins. Here, a peptide-based serological assay was developed to specifically detect exposure to MPXV. A comparative analysis of immunogenic proteins across human OPXVs identified a large subset of proteins that could potentially be specifically recognized in response to a MPXV infection. Peptides were chosen based upon MPXV sequence specificity and predicted immunogenicity. Peptides individually and combined were screened in an ELISA against serum from well-characterized Mpox outbreaks, vaccinee sera, and smallpox sera collected prior to eradication. One peptide combination was successful with ~86% sensitivity and ~90% specificity. The performance of the assay was assessed against the OPXV IgG ELISA in the context of a serosurvey by retrospectively screening a set of serum specimens from the region in Ghana believed to have harbored the MPXV-infected rodents involved in the 2003 United States outbreak.

Estimation of the Minimal Rift Valley Fever Virus Protective Neutralizing Antibody Titer in Human Volunteers Immunized with MP-12 Vaccine Based on Protection in a Mouse Model of Disease.

The Rift Valley fever virus (RVFV) MP-12 vaccine is a promising human and veterinary vaccine. Although the vaccine elicited neutralizing antibody (nAb) in human volunteers, the minimal antibody titer that is needed to afford protection is unknown. Therefore, this study was conducted to determine the minimal nAb titer elicited by the RVFV MP-12 vaccine in human volunteers that protected mice against lethal RVFV challenge as a surrogate assessment of the protective efficacy of the vaccine. Among volunteers who were vaccinated with the MP-12 vaccine during a phase II trial, sera with antibody titers of 1:20 collected 5 years post-vaccination (PV), 1:40 titer collected 2 years PV, and 1:80 titer collected 1 year PV was passively transferred to groups of BALB/c mice. Blood samples were obtained 1 day after passive transfer to determine the RVFV neutralizing nAb titer before challenge with pathogenic RVFV (strain ZH501). Our results indicated that 1 day after passive transfer of the immune sera, an approximate 4-fold reduction in circulating nAb titers was detected in the mice. The presence of RVFV nAb titers in the range of 1:5 to 1:20 were generally protective (75-100% survival). These results suggested that circulating titers of 1:5 or higher offer a high degree of protection by MP-12-elicited antibody in human volunteers. Also, the findings highlighted the value of using the BALB/c mouse RVFV challenge model as a surrogate for evaluating the protective nAb responses elicited by MP-12 and possible use for evaluating the efficacy of other RVFV vaccine candidates.

The O-Ag Antibody Response to Francisella Is Distinct in Rodents and Higher Animals and Can Serve as a Correlate of Protection.

Identifying correlates of protection (COPs) for vaccines against lethal human (Hu) pathogens, such as Francisella tularensis (Ft), is problematic, as clinical trials are currently untenable and the relevance of various animal models can be controversial. Previously, Hu trials with the live vaccine strain (LVS) demonstrated ~80% vaccine efficacy against low dose (~50 CFU) challenge; however, protection deteriorated with higher challenge doses (~2000 CFU of SchuS4) and no COPs were established. Here, we describe our efforts to develop clinically relevant, humoral COPs applicable to high-dose, aerosol challenge with S4. First, our serosurvey of LVS-vaccinated Hu and animals revealed that rabbits (Rbs), but not rodents, recapitulate the Hu O-Ag dependent Ab response to Ft. Next, we assayed Rbs immunized with distinct S4-based vaccine candidates (S4ΔclpB, S4ΔguaBA, and S4ΔaroD) and found that, across multiple vaccines, the %O-Ag dep Ab trended with vaccine efficacy. Among S4ΔguaBA-vaccinated Rbs, the %O-Ag dep Ab in pre-challenge plasma was significantly higher in survivors than in non-survivors; a cut-off of >70% O-Ag dep Ab predicted survival with high sensitivity and specificity. Finally, we found this COP in 80% of LVS-vaccinated Hu plasma samples as expected for a vaccine with 80% Hu efficacy. Collectively, the %O-Ag dep Ab response is a bona fide COP for S4ΔguaBA-vaccinated Rb and holds significant promise for guiding vaccine trials with higher animals.

Safety and immunogenicity of an inactivated eastern equine encephalitis virus vaccine.

BACKGROUND: Eastern equine encephalitis virus (EEEV) is a mosquito borne alphavirus spread primarily in Atlantic and Gulf Coast regions of the United States. EEEV is the causative agent of a devastating meningoencephalitis syndrome, with approximately 30% mortality and significant morbidity. There is no licensed human vaccine against EEEV. An inactivated EEEV vaccine has been offered under investigational new drug (IND) protocols at the United States Army Medical Research Institute of Infectious Diseases (USAMRIID) since 1976. METHODS: Healthy at-risk laboratory personnel received inactivated PE-6 strain EEEV (TSI-GSD 104) vaccine under two separate IND protocols. Protocol FY 99-11 (2002-2008) had a primary series consisting of doses on day 0, 7, and 28. Protocol FY 06-31 (2008-2016) utilized a primary series with doses on day 0 and 28, and month 6. Participants with an inadequate immune response, plaque reduction neutralization test with 80% cut-off (PRNT80) titer < 40, received booster vaccination. Volunteers with prior EEEV vaccination were eligible to enroll for booster doses based on annual titer evaluation. RESULTS: The FY06-31 dosing schema resulted in significantly greater post-primary series immune response (PRNT80 ≥ 40) rates (84% vs 54%) and geometric mean titers (184.1 vs 39.4). The FY 06-31 dosing schema also resulted in significantly greater cumulative annual immune response rates from 1 to up to 7 years post vaccination (75% vs 59%) and geometric mean of titers (60.1 vs 43.0). The majority of probably or definitely related adverse events were mild and local; there were no probably or definitely related serious adverse events. CONCLUSIONS: Inactivated PE-6 EEEV vaccine is safe and immunogenic in at-risk laboratory personnel. A prolonged primary series, with month 6 dose, significantly improved vaccine immunogenicity both post-primary series and longitudinally on annual titers. Despite decades of safe use under IND, full licensure is not planned due to manufacturing constraints, and ongoing development of alternatives.

Potent neutralization of Rift Valley fever virus by human monoclonal antibodies through fusion inhibition.

Rift Valley fever virus (RVFV), an emerging arboviral and zoonotic bunyavirus, causes severe disease in livestock and humans. Here, we report the isolation of a panel of monoclonal antibodies (mAbs) from the B cells of immune individuals following natural infection in Kenya or immunization with MP-12 vaccine. The B cell responses of individuals who were vaccinated or naturally infected recognized similar epitopes on both Gc and Gn proteins. The Gn-specific mAbs and two mAbs that do not recognize either monomeric Gc or Gn alone but recognized the hetero-oligomer glycoprotein complex (Gc+Gn) when Gc and Gn were coexpressed exhibited potent neutralizing activities in vitro, while Gc-specific mAbs exhibited relatively lower neutralizing capacity. The two Gc+Gn-specific mAbs and the Gn domain A-specific mAbs inhibited RVFV fusion to cells, suggesting that mAbs can inhibit the exposure of the fusion loop in Gc, a class II fusion protein, and thus prevent fusion by an indirect mechanism without direct fusion loop contact. Competition-binding analysis with coexpressed Gc/Gn and mutagenesis library screening indicated that these mAbs recognize four major antigenic sites, with two sites of vulnerability for neutralization on Gn. In experimental models of infection in mice, representative mAbs recognizing three of the antigenic sites reduced morbidity and mortality when used at a low dose in both prophylactic and therapeutic settings. This study identifies multiple candidate mAbs that may be suitable for use in humans against RVFV infection and highlights fusion inhibition against bunyaviruses as a potential contributor to potent antibody-mediated neutralization.

The Western Equine Encephalitis Lyophilized, Inactivated Vaccine: An Update on Safety and Immunogenicity.

Background: Western Equine Encephalitis (WEE) is a naturally acquired infection and potentially devastating bioweapon, with no specific human countermeasures. An experimental inactivated Western Equine Encephalitis Vaccine (WEEV; WEE TSI-GSD 210) has been used under an IND (investigational New Drug) protocol at the United States Army Medical Research Institute of Infectious Diseases (USAMRIID) since 1976. Methods: Over 24 years from 1987 to 2011, 876 subjects received 3 primary vaccine doses under 3 studies with 1,537 booster doses administered (FY87-8, phase 2, laboratory workers, vaccine lots 1-81-1, 1-81-2, and 2-1-91; FY99-12, phase 2 laboratory workers, lot 2-1-91; and FY09-02, phase 1 healthy volunteer, lot 3-1-92). Post-vaccination safety and immunogenicity [plaque reduction neutralization test 80% (PRNT80) > 1:40] were analyzed. Results: Overall PRNT80 response to the primary series in FY87-8 was 42% (326/770) but dropped to 16% (14/87) in FY99-12, prompting study FY09-02, which achieved 89% (17/19). The first booster response rate was 68% (814/1194) in FY87-8, 53% (171/324) in FY99-12, and 100% (10/10) in FY09-02. The majority of definitely related adverse reactions (AEs) were mild and local with no definitely related serious AEs. No laboratory acquired WEE infection was documented during this period despite 4 reported exposures in vaccinated subjects. Conclusion: The TSI-GSD 210 WEE vaccine was immunogenic, safe and well tolerated. Use of this vaccine could be considered in an emergency setting. Despite decades of safe and effective use under IND, full licensure is not planned due to manufacturing constraints, and a strategic decision to develop alternatives. Clinical Trial Registration: https://clinicaltrials.gov/, identifier NCT01159561.

Theoretical risk of genetic reassortment should not impede development of live, attenuated Rift Valley fever (RVF) vaccines commentary on the draft WHO RVF Target Product Profile.

In November 2019, The World Health Organization (WHO) issued a draft set of Target Product Profiles (TPPs) describing optimal and minimally acceptable targets for vaccines against Rift Valley fever (RVF), a Phlebovirus with a three segmented genome, in both humans and ruminants. The TPPs contained rigid requirements to protect against genomic reassortment of live, attenuated vaccines (LAVs) with wild-type RVF virus (RVFV), which place undue constraints on development and regulatory approval of LAVs. We review the current LAVs in use and in development, and conclude that there is no evidence that reassortment between LAVs and wild-type RVFV has occurred during field use, that such a reassortment event if it occurred would have no untoward consequence, and that the TPPs should be revised to provide a more balanced assessment of the benefits versus the theoretical risks of reassortment.

Retrospective Analysis of Pneumonic Tularemia in Operation Whitecoat Human Subjects: Disease Progression and Tetracycline Efficacy.

Francisella tularensis is a highly infectious Gram-negative bacterium that is the etiologic agent of tularemia in animals and humans. The incidence of tularemia is very low with a lack of comprehensive data that describe disease in humans due to difficulty in understanding time and routes of exposure. Under the title Operation Whitecoat, researchers at Ft. Detrick, MD conducted 40 clinical studies of tularemia from 1958 to 1968. In these studies, one of the objectives was to evaluate candidate countermeasures for treatment or prophylaxis of disease after exposure to Francisella tularensis strain Schu S4 by inhalation. These studies were reviewed retrospectively to delineate the early signs and symptoms or natural history of pneumonic tularemia and examine the efficacy of tetracycline in controlled human clinical studies. Using vital signs, onset of fever was objectively defined and calculated for each subject, while Adverse Events reported after exposure were also used to define the timing of disease onset and symptoms of early disease. There was a dose response relationship between time to fever onset and exposed dose at 200 cfu (172.8 h), 700 cfu (163.2 h), 2,500 cfu (105.3 h), and 25,000 cfu (75.5 h). Onset of fever was typically the earliest sign of disease at all doses but was often accompanied by symptoms such as headache, myalgia, chest pain, and nausea, irrespective of dose except at 200 cfu where only 50% of subjects exhibited fever onset or symptoms. Examining the efficacy of different treatment regimens of tetracycline, ineffective treatments were indicated by relapse of disease (fever and Adverse Events) after cessation of antibiotic treatment. Stratification of the data suggested that treatment for <14 days or doses <2g/day was associated with increased percentage of subjects with relapse of disease symptoms. Although these types of human challenge studies would not be ethically possible now, the climate post-World War II supported human testing under rigorous conditions with informed consent. Thus, going back and analyzing these unique clinical human challenge studies has helped describe the course of infection and disease induced by a biothreat pathogen and possible countermeasures for treatment under controlled conditions.

Phase 3 Efficacy Trial of Modified Vaccinia Ankara as a Vaccine against Smallpox.

BACKGROUND: Many countries have stockpiled vaccines because of concerns about the reemergence of smallpox. Traditional smallpox vaccines are based on replicating vaccinia viruses; these vaccines have considerable side effects. METHODS: To evaluate the efficacy of modified vaccinia Ankara (MVA) as a potential smallpox vaccine, we randomly assigned 440 participants to receive two doses of MVA followed by one dose of the established replicating-vaccinia vaccine ACAM2000 (the MVA group) or to receive one dose of ACAM2000 (the ACAM2000-only group). The two primary end points were noninferiority of the MVA vaccine to ACAM2000 with respect to the peak serum neutralizing antibody titers and attenuation of the ACAM2000-associated major cutaneous reaction by previous MVA vaccination, measured according to the maximum lesion area and the derived area attenuation ratio. RESULTS: A total of 220 and 213 participants were randomly assigned and vaccinated in the MVA group and ACAM2000-only group, respectively, and 208 participants received two MVA vaccinations. At peak visits, MVA vaccination induced a geometric mean titer of neutralizing antibodies of 153.5 at week 6, as compared with 79.3 at week 4 with ACAM2000 (a ratio of 1.94 [95% confidence interval {CI}, 1.56 to 2.40]). At day 14, the geometric mean titer of neutralizing antibodies induced by a single MVA vaccination (16.2) was equal to that induced by ACAM2000 (16.2), and the percentages of participants with seroconversion were similar (90.8% and 91.8%, respectively). The median lesion areas of the major cutaneous reaction were 0 mm2 in the MVA group and 76.0 mm2 in the ACAM2000-only group, resulting in an area attenuation ratio of 97.9% (95% CI, 96.6 to 98.3). There were fewer adverse events or adverse events of grade 3 or higher after both MVA vaccination periods in the MVA group than in the ACAM2000-only group (17 vs. 64 participants with adverse events of grade 3 or higher, P<0.001). CONCLUSIONS: No safety concerns associated with the MVA vaccine were identified. Immune responses and attenuation of the major cutaneous reaction suggest that this MVA vaccine protected against variola infection. (Funded by the Office of the Assistant Secretary for Preparedness and Response Biomedical Advanced Research and Development Authority of the Department of Health and Human Services and Bavarian Nordic; ClinicalTrials.gov number, NCT01913353.).

Rudolf Hess - The Doppelgänger conspiracy theory disproved.

The Deputy Führer of the Third Reich Rudolf Hess was captured after a controversial flight to Scotland in 1941. Hess was sentenced to life imprisonment during the Nuremberg War Crimes Trials. He was detained in Berlin's Spandau Prison under the official security designation 'Spandau #7.' Early doubts arose about the true identity of prisoner 'Spandau #7.' This evolved to a frequently espoused conspiracy theory that prisoner 'Spandau #7' was an imposter and not Rudolf Hess. After Hess's reputed 1987 suicide, the family grave became a Neo-Nazi pilgrimage site. In 2011, the grave was abandoned and the family remains cremated. Here we report the forensic DNA analysis of the only known extant DNA sample from prisoner 'Spandau #7' and a match to the Hess male line, thereby refuting the Doppelgänger Theory.

Human transcriptome response to immunization with live-attenuated Venezuelan equine encephalitis virus vaccine (TC-83): Analysis of whole blood.

Venezuelan equine encephalitis virus (VEEV) is an important human and animal alphavirus pathogen transmitted by mosquitoes. The virus is endemic in Central and South America, but has also caused equine outbreaks in southwestern areas of the United States. In an effort to better understand the molecular mechanisms of the development of immunity to this important pathogen, we performed transcriptional analysis from whole, unfractionated human blood of patients who had been immunized with the live-attenuated vaccine strain of VEEV, TC-83. We compared changes in the transcriptome between naïve individuals who were mock vaccinated with saline to responses of individuals who received TC-83. Significant transcriptional changes were noted at days 2, 7, and 14 following vaccination. The top canonical pathways revealed at early and intermediate time points (days 2 and 7) included the involvement of the classic interferon response, interferon-response factors, activation of pattern recognition receptors, and engagement of the inflammasome. By day 14, the top canonical pathways included oxidative phosphorylation, the protein ubiquitination pathway, natural killer cell signaling, and B-cell development. Biomarkers were identified that differentiate between vaccinees and control subjects, at early, intermediate, and late stages of the development of immunity as well as markers which were common to all 3 stages following vaccination but distinct from the sham-vaccinated control subjects. The study represents a novel examination of molecular processes that lead to the development of immunity against VEEV in humans and which may be of value as diagnostic targets, to enhance modern vaccine design, or molecular correlates of protection.

Povidone Iodine Ointment Application to the Vaccination Site Does Not Alter Immunoglobulin G Antibody Response to Smallpox Vaccine.

U.S. military personnel deployed to high-risk areas receive the live vaccinia virus (VACV) smallpox vaccine ACAM2000. VACV shedding from the vaccination site can result in autoinoculation and contact transmission. We previously found that the application of povidone iodine ointment (PIO) to the scarification site reduced viral shedding without altering the antibody response, as measured by plaque reduction neutralization or enzyme-linked immunosorbent assays. In this study, we used protein microarray assays to measure the amount of immunoglobulin G antibody bound to (1) ACAM2000 itself and (2) individual VACV antigens that are present within ACAM2000. We assessed antibody binding in sera from primary smallpox vaccinees who applied PIO to the scarification site beginning on day 7 (PIO group) and from those who did not apply PIO (control group). In both cohorts, the postvaccination antibody response-in terms of antibody binding, both to ACAM2000 and to 11 individual VACV antigens-was significantly greater than the prevaccination response (all p < 0.0001). The postvaccination antibody binding levels of vaccinees in the PIO group did not differ from those of control vaccinees. These findings further support the topical application of PIO, starting on day 7, to reduce the viral shedding associated with smallpox vaccination.

Safety and immunogenicity of a mutagenized, live attenuated Rift Valley fever vaccine, MP-12, in a Phase 1 dose escalation and route comparison study in humans.

Rift Valley fever (RVF) poses a risk as a potential agent in bioterrorism or agroterrorism. A live attenuated RVF vaccine (RVF MP-12) has been shown to be safe and protective in animals and showed promise in two initial clinical trials. In the present study, healthy adult human volunteers (N=56) received a single injection of (a) RVF MP-12, administered subcutaneously (SQ) at a concentration of 10(4.7) plaque-forming units (pfu) (SQ Group); (b) RVF MP-12, administered intramuscularly (IM) at 10(3.4)pfu (IM Group 1); (c) RVF MP-12, administered IM at 10(4.4)pfu (IM Group 2); or (d) saline (Placebo Group). The vaccine was well tolerated by volunteers in all dose and route groups. Infrequent and minor adverse events were seen among recipients of both placebo and RVF MP-12. One subject had viremia detectable by direct plaque assay, and six subjects from IM Group 2 had transient low-titer viremia detectable only by nucleic acid amplification. Of the 43 vaccine recipients, 40 (93%) achieved neutralizing antibodies (measured as an 80% plaque reduction neutralization titer [PRNT80]) as well as RVF-specific IgM and IgG. The highest peak geometric mean PRNT80 titers were observed in IM Group 2. Of 34 RVF MP-12 recipients available for testing 1 year following inoculation, 28 (82%) remained seropositive (PRNT80≥1:20); this included 20 of 23 vaccinees (87%) from IM Group 2. The live attenuated RVF MP-12 vaccine was safe and immunogenic at the doses and routes studied. Given the need for an effective vaccine against RVF virus, further evaluation in humans is warranted.

Rift Valley fever MP-12 vaccine Phase 2 clinical trial: Safety, immunogenicity, and genetic characterization of virus isolates.

An outbreak or deliberate release of Rift Valley fever (RVF) virus could have serious public health and socioeconomic consequences. A safe RVF vaccine capable of eliciting long-lasting immunity after a single injection is urgently needed. The live attenuated RVF MP-12 vaccine candidate has shown promise in Phase 1 clinical trials; no evidence of reversion to virulence has been identified in numerous animal studies. The objective of this Phase 2 clinical trial was to (a) further examine the safety and immunogenicity of RVF MP-12 in RVF virus-naïve humans and (b) characterize isolates of RVF MP-12 virus recovered from the blood of vaccinated subjects to evaluate the genetic stability of MP-12 attenuation. We found that RVF MP-12 was well tolerated, causing mostly mild reactions that resolved without sequelae. Of 19 subjects, 18 (95%) and 19 (100%) achieved, respectively, 80% and 50% plaque reduction neutralization titers (PRNT80 and PRNT50)≥1:20 by postvaccination day 28. All 18 PRNT80 responders maintained PRNT80 and PRNT50≥1:40 until at least postvaccination month 12. Viremia was undetectable in the plasma of any subject by direct plaque assay techniques. However, 5 of 19 vaccinees were positive for MP-12 isolates in plasma by blind passage of plasma on Vero cells. Vaccine virus was also recovered from buffy coat material from one of those vaccinees and from one additional vaccinee. Through RNA sequencing of MP-12 isolates, we found no reversions of amino acids to those of the parent virulent virus (strain ZH548). Five years after a single dose of RVF MP-12 vaccine, 8 of 9 vaccinees (89%) maintained a PRNT80≥1:20. These findings support the continued development of RVF MP-12 as a countermeasure against RVF virus in humans.