Table 2.

Comparison of neutralizing antibody titers using wt and chimeric challenge NA and SA EEEVs in PRNT90

Sample type/sample no.Virus strain inoculumNo. days from onset of illness to sample collectionNeutralizing antibody titera with the following challenge virus:
wt NA EEEV (NJ60)SINV/NA EEEVSINV/SA EEEVSINV (EgAr339)
Human serum, confirmed EEEV infection (n = 32)
    110,2405,120<80
    210,240640<80
    3b5,1202,560<80
    45,1201,280<80
    52,5601,28080
    62,5601,280<80
    72,560320<80
    81,280640160
    9–101,280640<80
    111,280320<80
    121,280160<20
    1364032020
    14–16640320<20
    1764080<20
    18320320<10
    19320160640
    2032016040
    21320160<20
    22–238040<10
    242010<10
    25–26b<10<10<10
Paired samplesc
27 (A, S1)6640160<20
28 (A, S2)135,1202,560320
29 (B, S1)716040<20
30 (B, S2)81,280320<80
31 (C, S1)92,5601,280<20
32 (C, S2)212,560320<80
Human serum, EEEV vaccinated (n = 22)
    33640640<10
    34–35640320<10
    36–3832080<10
    39–4016080<10
    41–4216040<10
    43–448040<10
    456020<10
    464010<10
    47–482010<10
    49–54<10<10<10
Human serum, no evidence of EEEV infection (n = 22), 55–76<10<10<10
Reference antibody controls (n = 12)
    77MHIAF (EEEV NJ60)d20,48010,24080
    78MHIAF (EEEV NJ60)20,48020,4802,560
    79MHIAF (EEEV BeAn5122, Brazil56)2,5602,5602,560
    80Equine sera (EEEV NJ60)d328,00082,0001,280
    81MHIAF (SINV Ar1055)<10<10<10320
    82MHIAF (SINV Ar1055)<10<10<10640
    83MHIAF (SINV 16260)<10<10<105,120
    84MHIAF (SINV EgAr339)<10<10<10160
    85(MHIAF/SINV EgAr339)d<10<10<1040,960
    86MHIAF (SINV Reed Warbler)<10<10<102,560
    87MHIAF (SINV Michalovce)<10<10<1020,480
    88Normal human control serum<10<10<10<10
  • a Neutralizing antibody titer is expressed as the reciprocal of the endpoint serum or reference antibody dilution that neutralized the challenge virus plaque count by 90%. The LLOQ was a neutralizing antibody titer of 10, with the exception of the samples challenged with SINV/SA EEEV, in which the LLOQ ranged from 10 to 80, depending on the sample volume remaining. <10 to <80, no detectable titer at the LLOQ.

  • b CDC/DVBD EEEV IgM ELISA positive-control sera collected during acute phase of illness.

  • c S1 and S2, samples 1 and 2, respectively.

  • d PRNT positive control.