Malaria infection diagnosed by PCR as a means of evaluating preerythrocytic candidate malaria vaccin

1
Malaria infection diagnosed by PCR as a means of
evaluating pre-erythrocytic candidate malaria
vaccines E. B. Imoukhuede, L. Andrews,
D. Nwakanma, P. Milligan, T. Berthoud, K. Bojang,
J. Ismaili, S. Keita, M. Sowe, C. Buckee, F.
Njie, S. Gilbert, T. Lang, B.M. Greenwood, A.V.S.
Hill
Background Heterolgous prime-boost immunisation
strategy using malaria antigens expressed in
plasmid or viral vectors has been shown to induce
strong cellular immune responses. DNA and viral
vaccines recombinant for a malarial DNA sequence
and designed to induce protective immunogenicity
against liver-stage P. falciparum malaria were
manufactured to explore this approach. Two
constructs have been used. The
circumsporozoite(CS) and the P.falciparum ME-TRAP
construct which includes a multiepitope
string(ME) and thrombospodin-related adhesion
protein(TRAP). Carriers for this construct
include plasmid DNA and recombinant poxviruses,
modified Vaccinia Ankara(MVA) and Fowlpox.
Background to Study A well-established sporozoite
challenge model has been used to assess
pre-erythrocytic vaccines in many non-immune
volunteers. The new technique involves repeated
blood sampling over a short period of time to
detect by PCR low levels of malaria infection
that cannot be detected by microscopy. It is
uncertain how well this model predicts field
efficacy and few centres have access to this
method of testing. Field trials of the efficacy
of candidate malaria vaccines currently require
hundreds of volunteers. The ability to test
candidate pre-erythrocytic vaccines in a field
setting with group sizes of tens rather than
hundreds could greatly facilitate identification
of the most promising vaccine candidates
NUMBER OF VOLUNTEERS SEEN FOR DAILY FINGER
PRICKS

• Objectives
• To determine if the PCR technique could be an
  economical method of undertaking field evaluation
  of pre-erythrocytic candidate malaria vaccines
• To determine if the very intensive blood sampling
  that this method requires will be acceptable

• Study setting/design
• 102 volunteers enrolled from 9 villages east of
  Farafenni town, The Gambia (FDSS).
• All groups received one dose of Primaquine (30mg)
  7 days before the last vaccination
• All groups received 3 day course of
  Lapdapartesunate starting on day of last
  vaccination
• SP Group only received one dose of Fansidar on
  the day before follow-up commenced
• Follow up by daily finger pricks to obtain 0.5
  mls of blood for PCR analysis/ duplicate blood
  film for a 28-day period
• Daily follow-up started 7 days after final
  vaccination
• Study duration 4 months
• Randomisation groups
• Group 1(Malaria vaccine) Received 2 doses of
  the malaria vaccine FP9 ME-TRAP and 1 dose of MVA
  ME-TRAP at 4 week intervals
• Group 2 (SP) Received 3 doses of rabies vaccine
  at 4 week intervals
• Group 3(Rabies) same as Group 2

• Results
• Vaccines were safe and no SAEs were reported.
• In contrast to previous studies, lower
  immunogenicity was observed with malaria vaccines
  used in this study
• No significant difference in time to parasitaemia
  was observed in efficacy analysis (comparing
  Malaria Vaccine and Rabies groups)
• An average of over 70 of volunteers were sampled
  daily during the 28-day follow-up period
• Unexpectedly, we found no differences in the rate
  of development of low-level infections in the SP
  group, compared to the malaria vaccine and
  rabies groups during the surveillance period
• Likely caused by parasites released from liver
  schizonts before they were eliminated by SP
• We suggest that this low-density parasitaemia,
  previously undetected in trials using microscopy,
  could have been intermittently cleared from the
  blood by circulating SP thereby generating some
  protective blood stage immunity.