key: cord-0047827-wt5xm1nr
authors: Roman, François P.; Coccia, Margherita; Schuerman, Lode
title: Commentary in reply to a publication on Plasmodium falciparum pre-erythrocytic stage vaccine development
date: 2020-07-20
journal: Malar J
DOI: 10.1186/s12936-020-03334-1
sha: 3983b80d7b1e892d38819ec1ac20fd006d46c33d
doc_id: 47827
cord_uid: wt5xm1nr

We have read the publication of Molina-Franky and colleagues on Plasmodium falciparum pre-erythrocytic stage vaccine development (Malaria Journal, 2020;19:56). The commentary revises some of their statements on the RTS,S/AS01 vaccine that are considered either imprecise or incorrect.

While we agree that efforts to find and introduce new candidate pre-erythrocytic malaria vaccines are warranted, we disagree with the statement of Molina-Franky and colleagues [1] that none of these approaches has led to promising results regarding an effective control of the disease. It is indeed our opinion that the results available today for RTS,S/AS01, and based on at least 28 clinical Phase I, II and III studies [2] (and not five studies as suggested in Table 1 of the paper) do indicate the potential of this vaccine to provide substantial public health benefit. This is well supported by the European Medicine Agency (EMA) (as reflected in their positive scientific opinion [3]), the World Health Organization (WHO) (as reflected in the WHO position paper on this vaccine [4] ) and the national regulatory authorities in Ghana, Kenya and Malawi [5] .

We would also like to address some of the specific points raised by the paper. The authors state that "A tetrapeptide from the CSP NANP tandem repeat region (R) and the C-terminal region containing T cell (T) epitopes (exclusive for the NF54 strain) are fused to the hepatitis B surface (S) antigen". However, each RTS molecule includes 19 copies of the tetrapeptide repeat motif (NANP). In addition, the fact that the C-terminal region of the CSP protein included in the vaccine is derived from the NF54 parasite strain does not mean that it is exclusive for that strain [6] [7] [8] .

With reference to the WHO position paper on Malaria vaccine [4] , the authors state that the EMA issued a "cautious scientific opinion regarding its quality". However, neither the WHO position paper, nor EMA, qualified the positive scientific opinion as cautious, and no specific caution was expressed on the quality of the RTS,S/AS01 vaccine in particular.

The authors also address a series of other concerns relating to the profile of RTS,S/AS01 or to some of its components, such as the genetic variability of the selected CSP region, high parasitaemia levels in "individuals considered protected", presumed pro-apoptotic signals induced by some RTS,S components, or a lack of mechanistic understanding of the AS01 adjuvant system used in the vaccine.

Regarding CSP variability, we know that in the large phase III trial where efficacy was demonstrated against malaria, fewer than 10% of the parasites matched the CSP protein alleles used in the vaccine [9] . These reassuring data clearly mitigate the concern that CSP variability may neutralize vaccine efficacy. High parasitaemia levels (i.e. > 5000 parasites/μl or 0.1% parasitaemia) are not unexpected after RTS,S vaccination, since the objective of the vaccination is not to prevent infection but to reduce the risk of clinical episodes of malaria. During RTS,S vaccine development the > 5000 parasites/μl threshold was used to define clinical cases of malaria to evaluate vaccine efficacy, but not as a direct efficacy surrogate of the vaccine [10] [11] [12] [13] [14] . A child with a parasitaemia of 5000 parasites/µl or more was not considered to be protected, but on the contrary to experience a malaria episode.

It is unclear why the authors attribute pro-apoptotic signals to RTS,S components, based on the references provided in the manuscript [15, 16] . From our own review on the two cited papers, such pro-apoptotic signals are described as potential malaria disease-related mechanisms, or as B-cell based mechanisms in the context of influenza vaccination, but not as RTS,S/AS01related mechanisms. Importantly, the actual clinical implications of this statement are unclear.

Finally, albeit the mechanism of action of AS01 is indeed complex, significant published work exists now that clarify many of its aspects [17, 18] , including the mechanisms of QS21 when formulated in liposomes [19, 20] .

Plasmodium falciparum pre-erythrocytic stage vaccine development

Clinical development of RTS, S/AS malaria vaccine: a systematic review of clinical Phase I-III trials

Malaria vaccine: WHO position paper

WHO. Q&A on the Malaria Vaccine Implementation Pilot (MVIP

Malaria vaccines in development

Development of the RTS, S/AS malaria candidate vaccine

From the circumsporozoite protein to the RTS,S/AS candidate vaccine

Genetic diversity and protective efficacy of the RTS, S/AS01 malaria vaccine

Design of a phase III multicenter trial to evaluate the efficacy of the RTS, S/AS01 malaria vaccine in children across diverse transmission settings in Africa

First results of phase 3 trial of RTS,S/ AS01 malaria vaccine in African children

A phase 3 trial of RTS,S/AS01 malaria vaccine in African infants

Efficacy and safety of the RTS,S/ AS01 malaria vaccine during 18 months after vaccination: a phase 3 randomized, controlled trial in children and young infants at 11 African sites

Efficacy and safety of RTS,S/AS01 malaria vaccine with or without a booster dose in infants and children in Africa: final results of a phase 3, individually randomised, controlled trial

T cell-mediated immunity to malaria

Enhancement of adaptive immunity by the human vaccine adjuvant AS01 depends on activated dendritic cells

Cellular and molecular synergy in AS01-adjuvanted vaccines results in an early IFNγ response promoting vaccine immunogenicity

Central role of CD169 + lymph node resident macrophages in the adjuvanticity of the QS-21 component of AS01

Lysosome-dependent activation of human dendritic cells by the vaccine adjuvant QS-21

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