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The gold-standard method for quantifying neutralizing antibody responses to many viruses, including dengue virus (DENV), is the plaque reduction neutralization test (PRNT, also called the immunofocus reduction neutralization test). The PRNT conducted on 96-well plates is high-throughput and requires a smaller volume of antiserum than on 6- or 24-well plates, but manual plaque counting is challenging and existing automated plaque counters are expensive or difficult to optimize. We have developed Viridot (Viridot package), a program for R with a user interface in shiny, that counts viral plaques of a variety of phenotypes, estimates neutralizing antibody titers, and performs other calculations of use to virologists. The Viridot plaque counter includes an automatic parameter identification mode (misses <10 plaques/well for 87% of diverse DENV strains [n = 1521]) and a mode that allows the user to fine-tune the parameters used for counting plaques. We compared standardized manual and Viridot plaque counting methods applied to the same wells by two analyses and found that Viridot plaque counts were as similar to the same analyst's manual count (Lin's concordance correlation coefficient, ρc = 0.99 [95% confidence interval: 0.99-1.00]) as manual counts between analysts (ρc = 0.99 [95% CI: 0.98-0.99]). The average ratio of neutralizing antibody titers based on manual counted plaques to Viridot counted plaques was 1.05 (95% CI: 0.98-1.14), similar to the average ratio of antibody titers based on manual plaque counts by the two analysts (1.06 [95% CI: 0.84-1.34]). Across diverse DENV and ZIKV strains (n = 14), manual and Viridot plaque counts were mostly consistent (range of ρc = 0.74 to 1.00) and the average ratio of antibody titers based on manual and Viridot counted plaques was close to 1 (0.94 [0.86-1.02]). Thus, Viridot can be used for plaque counting and neutralizing antibody titer estimation of diverse DENV strains and potentially other viruses on 96-well plates as well as for formalization of plaque-counting rules for standardization across experiments and analysts., Competing Interests: The authors have declared that no competing interests exist.

Unlabelled: The ideal dengue vaccine will provide protection against all serotypes of dengue virus and will be economical and uncomplicated in its administration. To determine the ability of a single dose of the live attenuated tetravalent dengue vaccine TV003 to induce a suitable neutralizing antibody response, a placebo-controlled clinical trial was performed in 48 healthy adults who received 2 doses of vaccine or placebo administered 12 months apart. Evaluation of safety, vaccine viremia, and neutralizing antibody response after each dose indicated that the first dose of vaccine was capable of preventing infection with the second dose, thus indicating that multiple doses are unnecessary., Clinical Trials Registration: NCT01782300., (Published by Oxford University Press for the Infectious Diseases Society of America 2016. This work is written by (a) US Government employee(s) and is in the public domain in the US.)