At present, there are a total of 64 survey areas selected for the vector surveillance throughout the territory and the following four different indices will be enumerated:

1. Area Gravidtrap Index for Aedes albopictus (AGI),
2. Area Density Index for Aedes albopictus (ADI),
3. Monthly Gravidtrap Index for Aedes albopictus (MGI), and
4. Monthly Density Index for Aedes albopictus (MDI).

The AGI and ADI indicate the extensiveness of the distribution and density of Aedine mosquitoes respectively in that particular survey area, while the MGI and MDI are enumerated by pooling together all AGIs and ADIs of the same month reflecting the territory-wide situation of Aedes albopictus.

In addition to the areas in the community, surveys of Aedes albopictus are also carried out to cover all major port areas. The following two different groups of indices will be enumerated:

1. Port / Port Monthly Gravidtrap Index for Aedes albopictus (PGI/PMGI), and
2. Port / Port Monthly Density Index for Aedes albopictus (PDI/PMDI).

The Port Gravidtrap Index for Aedes albopictus (PGI) indicates the extensiveness of the distribution of Aedine mosquitoes in a particular group of port areas. The Port Monthly Gravidtrap Index for Aedes albopictus (PMGI) is enumerated by pooling together all PGIs of the same month, which reflects the overall degree of occurrence of Aedes albopictus in port areas. Another index, the Port Density Index for Aedes albopictus (PDI) indicates the density of Aedes albopictus in positive traps in a particular group of port areas. The Port Monthly Density Index for Aedes albopictus (PMDI) is enumerated by pooling together all PDIs of the same month, which denotes the overall density of Aedes albopictus in locales detected with Aedine mosquito activities in port areas.

The period for setting Gravidtraps in the survey areas is two weeks. During the survey period, the Gravidtraps are collected on a weekly basis. At the end of the first week of surveillance, all the Gravidtraps are retrieved and another batch of Gravidtraps is set at the same location for surveillance for the second week. The glue boards inside the retrieved Gravidtraps are immediately examined for the presence of adult Aedine mosquitoes (“Aedes-positive”) for the purpose of compiling the Gravidtrap Index (First Phase) and Density Index (First Phase). At the end of the second week of surveillance, all the Gravidtraps are retrieved and instantly checked for the presence of adult Aedine mosquitoes (“Aedes-positive”) on the glue boards. Data of the two weeks’ surveillance will be combined to obtain the Area Gravidtrap Index (AGI) and Area Density Index (ADI).

FEHD will notify its District Environmental Hygiene Offices and other relevant government departments of the Gravidtrap Indices and Density Indices of various phases once the figures are available, so that they can promptly carry out targeted mosquito prevention and control work. To facilitate public access to the latest survey data, FEHD will announce the phased indices for Aedes albopictus for each of the surveyed areas on the FEHD webpage. Corresponding mosquito prevention and control measures can be taken by the public in a more expeditious manner so as to prevent the spread of dengue fever.

The Gravidtrap Index and Density Index for Aedes albopictus of each surveyed area are released in two phases. Figures released in the first phases are phased indices and the second phase indices represent the AGI and ADI of the surveyed area in that month.

The Gravidtrap Index for Aedes albopictus is divided into four levels, reflecting the distribution level of the Aedes albopictus. Level 1 (less than 5 per cent) indicates that distribution of Aedes albopictus is not extensive in the area surveyed. Level 2 (5 per cent to less than 20 per cent) indicates that distribution of Aedes albopictus is fairly extensive in the area surveyed. Level 3 (20 per cent to less than 40 per cent) indicates that distribution of Aedes albopictus is extensive in the area surveyed. Level 4 (40 per cent or above) indicates that distribution of Aedes albopictus is very extensive in the area surveyed. Specific preventive and control measures will be initiated accordingly. Management of housing estates, hospitals, schools, construction sites and other venues or properties are also advised to take specific measures to contain mosquito problems in their own properties. The four levels of Gravidtrap Index for Aedes albopictus and actions to be taken by the management are listed in the following table.

With the adoption of Gravidtraps since April 2020, a new index called the Density Index (DI) is also recorded in each survey area.  It is denoted as Area Density Index (ADI) which is enumerated by dividing the number of adult Aedes by the total number of Aedes positive Gravidtraps during the survey period.  The minimum value of the index is one.

The Density Index for Aedes albopictus is divided into three levels, reflecting the abundancy of Aedes albopictus.  Level 1 (1 to 1.5) indicates that mosquito is not abundant in the locations with positive Gravidtraps.  Level 2 (1.6 to 2.4) indicates that mosquito is slightly abundant in the locations with positive Gravidtraps.  Level 3 (2.5 or above) indicates that mosquito is abundant in the locations with positive Gravidtraps.  Specific preventive and control measures will be initiated accordingly.  Management of housing estates, hospitals, schools, construction sites and other venues or properties will also be advised to take specific measures to contain mosquito problems in their own properties.  The three levels of DI and actions to be taken by the management are listed in the following table. 

Public should refer to the “Action to be taken” for AGI as primary reference.  If both indices are of similar levels, their actions are also similar.  However, if the AGI is low, e.g. Level 1 or Level 2, but level of ADI is high, e.g. Level 3, public should carry out actions for corresponding level of AGIs and supplement with actions for higher level of ADI, e.g. fogging, in locations with high mosquito density.

The phased Gravidtrap Indices and Density Indices for Aedes albopictus for each surveyed area in May 2023 are provided below:

Remarks:
Gravidtrap Index figures in green:                Level I : Gravidtrap Index <5.0% (indicates that the distribution of Aedes albopictus in the survey area is not extensive)
Gravidtrap Index figures in yellow:               Level II : 5.0% ≤ Gravidtrap Index <20.0% (indicates that the distribution of Aedes albopictus in the survey area is fairly extensive)
Gravidtrap Index figures in red:                     Level III : 20.0%≤  Gravidtap Index <40.0% ( indicates that the distribution of Aedes albopictus in the survey area is extensive)
Gravidtrap Index figures in purple:                Level IV : Gravidtrap Index ≥40.0% (indicates that the distribution of Aedes albopictus in the survey area is very extensive)

Density Index figures in green:                      Level I : 1.0 ≤ Density Index ≤ 1.5 (Aedes albopictus is not abundant in the locations with positive Gravidtraps)
Density Index figures in yellow:                     Level II : 1.6 ≤ Density Index ≤ 2.4 (Aedes albopictus is slightly abundant in the locations with positive Gravidtraps)
Density Index figures in red:                          Level III : Density Index ≥ 2.5 (Aedes albopictus is abundant in the locations with positive Gravidtra