Dengue virus is a single-stranded positive-sense RNA virus of the family Flaviviridae, genus Flavivirus, primarily transmitted by Aedes mosquitoes. Four DENV serotypes have been identified; all can cause the full spectrum of disease [21]. WHO estimates there are 50–100 million cases and over 20,000 deaths due to dengue infection annually [22].

Dengue outbreaks of varying duration and serotype have been reported in the PICs since the late 1800s [18,19]. Historically, dengue outbreaks in PICs have had a cyclic pattern with one serotype dominating, being replaced every 3–5 years [19]; however, in 2014 Dupont-Rouzeyrol et al. reported this pattern had changed with co-circulation of serotypes becoming common [23]. Roth et al. (2014) identified 18 outbreaks of different serotypes occurring over the 2012–2014 period including, for the first time on record, all four DENV serotypes circulating at once [20,24]. Exposure to one serotype of dengue does not result in immunity to another serotype, leaving populations vulnerable to reinfection [19]. There is further evidence that those who have been infected with multiple serotypes are at increased risk of severe dengue infection [25,26].

During the study period, dengue was the most commonly reported arboviral disease; almost all (19 out of 22) PICs reported at least one dengue outbreak event. In total, our review identified 72 DENV outbreaks—15 DENV-1, 20 DENV-2, 14 DENV-3, four DENV-4, and an additional 19 outbreaks with undetermined serotype—suggesting the pattern identified by Dupont-Rouzeyrol et al. of co-circulation has continued (Figure 2).

All four dengue serotypes were co-circulating across the Pacific Island region in 2016, 2017, and 2018; and DENV-1, DENV-2, DENV-3 and DENV-unspecified were co-circulating in 2014, 2015, 2019 and 2020 (Figure 2). In the study period, six PICs recorded co-circulation of two or more DENV serotypes (Cook Islands [27], Fiji [27,28], French Polynesia [27,29], New Caledonia [27,30,31], PNG [27,28,31,32,33,34], and Solomon Islands [27,28,31,33]; and two confirmed co-circulations of all four DENV serotypes (PNG 2016 [27,28,31,32,33,34], New Caledonia 2018 [27,28]).

Dengue outbreaks ranged from less than 10 to more than 12,250 cases (due to limited testing capacity, suspected case numbers were often reported). New Caledonia was the PIC that reported the most outbreaks (n = 13) (Figure 2) [27,28,30,31].

Of the 72 dengue outbreaks reported, case numbers were available for 55. At least 1000 cases were reported in 18 outbreaks, and 16 outbreaks had less than 100 cases.

During the study period, the largest dengue outbreak on record in the PICs occurred in the Solomon Islands in 2016/17. This outbreak is reported to have affected at least 12,329 people, resulting in 877 hospitalizations and 16 deaths [27,31,33]. Another notable outbreak occurred in Guam in 2019, which although small (18 cases), is considered important as it was the first autochthonous outbreak of the disease in Guam in 75 years [27].

PNG reported two separate periods of co-circulation of four and then three DENV serotypes in 2016 and 2018, respectively. The first episode of co-circulating dengue was from January to June of 2016 [27,28,31,32,33,34], followed by cases of DENV-1, DENV-2, DENV-4, and DENV-unspecified in 2018 (Figure 2) [27]. Curiously, no outbreaks were reported in PNG outside of these times. This may suggest periods of no dengue circulation; however, this seems unlikely and contradicts more recent evidence suggesting the persistent circulation of DENV-1, DENV-2, and DENV-3 in PNG for over a decade [32,33]. This is significant as endemic PNG strains serve as a potential reservoir for the spread of the virus to other countries, as demonstrated by molecular epidemiology studies that have linked outbreaks in Australia, Solomon Islands, and Fiji [32,35]. Further research is needed to understand the circulation of dengue in PNG, including elucidating the genetic diversity of strains and the potential risk for their regional dissemination.

Chikungunya virus is transmitted to humans by the bite of an infected Aedes mosquito, most commonly Ae. aegypti and Ae. albopictus, although Ae. polynesiensis can also transmit the virus [40,41]. Chikungunya infections usually manifest as a self-limiting dengue-like illness with high fever, severe arthralgias, myalgias, and maculopapular rash. Rare but severe complications may occur [2,41].

Roth et al. reported seven chikungunya outbreaks between 2012 and 2014 [20]. We found 14 outbreaks in the study period affecting 11 PICs: three outbreaks in Fiji [42], two in New Caledonia [27], and one in each of America Samoa [27], Samoa [27], Tokelau [27], French Polynesia [27], Cook Islands [27,29], Kiribati [27], Marshall Islands [27], Nauru [27], and Tuvalu [27] (Figure 4).

While half of these events resulted in relatively small outbreaks (affecting <100 people), there were also notable epidemics. The most striking occurred in French Polynesia from October 2014 to March 2015, affecting an estimated 66,000 people (estimated attack rate = ~25%) [43], and resulted in 64 admissions to intensive care and 18 deaths [44]. Health authorities observed a four- to nine-fold increase in Guillain–Barre syndrome (GBS) cases at the time, raising suspicion of a causal relationship with chikungunya infection. This was the second suspected arbovirus-triggered outbreak of GBS in French Polynesia, following Zika-associated cases in 2013/14 [45]. American Samoa (2500 cases) [46], Samoa (4524 cases) [27], the Cook islands (782 cases) [27], and Tokelau (200 cases) [47] experienced chikungunya outbreaks at a similar time raising suspicion of regional spread; however, this was disproven by genomic analysis that found the French Polynesian outbreak was epidemiologically linked to the Caribbean, suggesting the infection was imported by a traveler [43].

Other notable chikungunya outbreaks were reported in Kiribati (13,309 cases) [27], the Marshall Islands (1317 cases) [27], New Caledonia, (est. 58 cases) [27], and Fiji (86 cases) [42]. Cases were also identified in Queensland (Australia), and New Zealand among travelers recently returned from a PIC [27].

A serological survey conducted in Fiji in 2013 by Kama et al. soon after the first chikungunya cases were detected in the country found population antibody seroprevalence rates to be low (<1%), suggesting minimal exposure had occurred and that the population was immunologically naïve to CHIKV [48]. A follow-up serosurvey conducted in June 2017 (after two outbreaks in Fiji) found a seroconversion rate of 15.5% [42].

First isolated from a non-human primate in 1947 in Africa, human Zika infections had only been reported in 14 human cases (all in Africa) until 2007 [50] when the first documented large outbreak of the disease occurred in Yap State, Federated States of Micronesia [51]. Subsequently, Zika has spread to other PICs through international travelers and trade, with Roth et al. reporting three outbreaks (in Cook Islands, New Caledonia and French Polynesia) between 2012 and 2014 [20].

Our review identified 18 Zika outbreaks affecting 14 PICs, with four countries experiencing two outbreak events during the study period (American Samoa 2016/17 and 2018, RMI 2016 and 2017, New Caledonia 2014/15 and 2016, Solomon Islands 2015 and 2016). Zika outbreaks in Tonga 2016 [27,28,33,38,39], New Caledonia 2014/15 [27,52,53], and American Samoa 2016/17 [27,28,33,39,54,55] reported relatively high case counts of 2420, 1500, and 1004, respectively (Figure 5).

Zika-associated cases of microcephaly among newborn children in French Polynesia in 2013–2014, and subsequent Zika-associated GBS in South America in late 2015/early 2016 led World Health Organization of a ‘public health emergency of international health concern’ [3,4], triggering enhanced public health and clinical response measures globally.

In 2019, Sheel et al. linked the emergence of Zika in PICs to environmental stresses caused by climate change and associated pressures on fragile public health systems [56]. Craig et al. (2017) tested a hypothesis that in the absence of a dedicated surveillance system, routinely collected data on the incidence of acute flaccid paralysis (AFP), a cardinal sign of GBS, may provide a useful early warning indicator for Zika-related neurological complications. However, the authors found no conclusive evidence to support this hypothesis [39].

The review did not find definitive evidence of other arboviral outbreaks; however, the likely circulation of Ross River Virus (RRV) in French Polynesia [60] and Barmah Forest Virus in PNG [61] was mentioned in the literature.

The suspicion of RRV circulation was supported by Aubry et al. (2017), which presents serological evidence of circulation among the French Polynesian population before 2014 [62]. Lau et al. (2017) suggest that diagnosing RRV, if present, would be unlikely as most PICs lack the capacity to perform serology to confirm infection with the virus [63]. Further, symptomatic surveillance would likely lack sensitivity given the similarity in symptom profiles with other more expected arboviral infections. With 55% to 75% of RRV infections being asymptomatic, symptom-based surveillance methods are likely inadequate [60].

A report by Caly et al. (2019) found evidence that Barmah Forest virus, a virus that has never been detected outside Australia, was circulating in PNG [61].