Clinical samples were collected from 514 total patients, of which 207 (40%) tested positive by ZIKV rRT-PCR or ZIKV IgM or both. Nine samples tested positive by both tests. Testing sera from 480 patients resulted in 107 positives for ZIKV by rRT-PCR and 109 positives for ZIKV IgM. Of the 102 urine samples, 12 tested positive for ZIKV by rRT-PCR. The one CSF sample tested negative by rRT-PCR.
 Table 1 summarizes the spatial and temporal occurrences of reported cases, the number of symptomatic and asymptomatic cases, the number of positive and negative cases for each parish and for the country. Also shown is the number of Guillain-Barre syndrome (GBS) cases identified by the Grenada Ministry of Health. The parishes on the main island are listed in decreasing order of population size with the neighboring island of Carriacou listed last. The parish population data are based on the 2011 Grenada Census data.
The outbreak began in the last week of April in St. Andrew Parish when a 28-year-old female tested positive for ZIKV by rRT-PCR. Her reported symptoms included rash, fever, headache, joint pain, body pain, lymphadenopathy, nausea or vomiting, and diarrhea. This patient is considered the index case for the country.
To make relative comparisons among geographic areas across the country, an indicated attack rate was calculated for each parish as the proportion of positive samples collected from a parish scaled to the size of the parish. With a population of 25,722 people and 36 positive cases, St. Andrew Parish had an indicated attack rate of 14 cases of ZIKV infection per 10,000 people. Of the 36 positive cases, 18 were rRT-PCR positive and 18 were ZIKV IgM positive. The positive cases collected in St. Andrew Parish were 17% of the total ZIKV-positive cases identified in this study.
St. Mark Parish, in the northwest of the country, was the next to report its first case, during the week of May 8. St. Mark Parish has a population of 4,086 with 12 positive cases reported, of which eight tested rRT-PCR positive and four tested ZIKV IgM positive, giving an indicated attack rate of 29 positive cases per 10,000 in the population. St. Mark Parish had 6% of the ZIKV-positive cases identified in this study.
St. George Parish, in the southwest, with a population of 36,823, was the next to report its first case in late May. With 117 positive cases of which 66 were rRT-PCR positive and 51 were ZIKV IgM positive, St. George Parish had an indicated attack rate of 32 per 10,000 people in the population. St. George Parish had 57% of the ZIKV-positive cases identified in this study. With the highest population density in the country, St. George Parish was also the focus of infection during the outbreaks of DENV in 2001-2002 and CHIKV in 2014.
After St. George Parish, three other parishes reported their first cases within two weeks of each other: St. Patrick Parish in the week of June 5, St. David Parish in the week of June 12, and St. John Parish in the week of June 26. These three parishes combined had 16% of the total positive cases identified in this study.
Carriacou, located northeast off the coast of St. Patrick Parish, and accessible only by a ferry or plane, was the last parish to have its first diagnosed case of ZIKV, which was reported during the week of July 3. Carriacou had only 4% of the total positive cases identified in this study.
The countrywide-indicated attack rate during the outbreak was 20 per 10,000 people with peak transmission occurring in the week of June 26 with 26 cases, of which 21 were rRT-PCR positive and 5 were ZIKV IgM positive. The reported date of symptom onset was used as the week of indicated infection. When the symptom onset date was not available, the date of sample collection was used as a conservative infection date.
 Figure 1 shows the spatial distribution of all ZIKV-positive cases with the highest concentration in St. George Parish in the southernmost area, which is the most urban area.
 Figure 2 presents graphically the temporal distribution of ZIKV-positive cases during the outbreak by week. The rRT-PCR-positive cases are shown in dark blue and the ZIKV IgM-positive cases are shown in light blue with the GBS-positive cases identified by the Grenada Ministry of Health shown in green.
The ZIKV vector, A. aegypti, is known to be an endophagic urban dweller that clusters around population centers. In past outbreaks in Grenada of other arboviruses carried by the same mosquito species, specifically the DENV (2001-2002) and CHIKV (2014) outbreaks, cases tended to cluster in population centers. This was observed again during the ZIKV (2016) outbreak.
St. George Parish, which is the most densely populated parish with 36% of Grenada's population, had 57% of the positive cases identified in this study. St. Andrew Parish with 25% of the population had 17% of the reported positive cases. St. Mark Parish and St. George Parish, despite having, respectively, the lowest and highest populations, had the highest indicated attack rates. It is reasonable to conjecture that this could be because these parishes had early cases of the disease, and thus, there was ample time for the virus to circulate within their immunologically naive populations.