In Hanoi dengue transmission demonstrates clear annual cycles that are associated with a lag of around two months with seasonal increases in mean temperatures and rainfall. This pattern reflects climate driven changes in vector abundance, survival, and biting frequency; and the time it takes for a newly infected mosquito to become infectious to humans (the extrinsic incubation period) 
. We observed a pattern of high wind speeds being associated with periods of low dengue notifications, an association noted by other authors 
. Although it is not established whether this association is causal, high wind speed could conceivably interfere with normal mosquito movements and biting behaviors.
Large outbreaks occurred in 1998 and 2009, and wavelet analysis indicated possible 2–3 years periodicity. A 2–4 years cycle has been described in Thailand, Cambodia, and South Vietnam. Moreover, a meta-cycle between 8–10 years has been observed in Thailand 
. This cycle of larger multi-year outbreaks superimposed on an annual cycle is a characteristic feature of dengue epidemiology that has been attributed to various models of serotype-host interactions, and a possible influence of multi-year climate oscillations 
. With a peak of 384 total notified cases per 100,000 persons in 2009, Hanoi has a much lower dengue incidence than other parts of Southeast Asia. Ho Chi Minh City in South Vietnam had around 199 hospitalized cases per 100,000 in 2009, whilst in Thailand and Cambodia the rates of symptomatic dengue infection in children <15 years are estimated at around 24/1000 and 41/1000 respectively 
. Yet despite these large differences in incidence, similar temporal patterns are observed in Hanoi, suggesting that the intensity of dengue transmission in Hanoi may be sufficient to replicate the inter-subtype dynamics observed in higher transmission areas 
. This will have relevance for understanding how dengue vaccination may influence intrinsic pathogen-host dynamics in settings with different intensities of transmission 
Excluding the two major outbreaks (in 1998 and 2009), the annual age adjusted incidence of notified dengue increased significantly. However, over the same period the average age of infection has not decreased, as would be expected if the per capita rate of infection in susceptible people (the force of infection) was increasing 
. This apparent discordance could arise through several mechanisms: 1) the recorded increase in incidence may be an artifact of improved case detection and reporting; 2) unmeasured demographic changes, such as an influx of unregistered adult workers, may increase the average age of the population beyond that captured by census and population estimates; 3) susceptible adult in-migration from areas of low dengue risk may increase the relative abundance of susceptible adult individuals; 4) the force of infection has declined resulting in fewer infections and an increase in the median age of infection, but this has resulted in an increase in the incidence of clinical cases (as opposed to infections) since adults are more likely than children to suffer clinically apparent illness as a result of primary dengue infection 
. It is not possible to distinguish between these scenarios on the basis of passively notified clinical dengue illness alone, and population-based studies with carefully monitored demographics and serial seroepidemiology are needed.
The average age of notified dengue cases in Hanoi is high and 85% are aged over 15 years. Although the average age of clinical cases in Thailand has increased over recent years, the clinical burden of dengue in much of Southeast Asia still primarily falls on children aged less than 15 years 
. Dengue does however predominantly affect adults in several Southeast Asia countries including Malaysia, Sri Lanka, and Singapore, and dengue is an adult problem in much of the Americas 
. The most important influences on geographical differences in the age group predominantly affected are likely to be the local intensity of dengue transmission and the time since dengue was (re)introduced into the population 
. The high average age of notified dengue cases in Hanoi may reflect a relatively low force of infection but may also reflect relatively recent introduction of dengue into Hanoi such that the adult population has not yet acquired multitypic immunity 
Our study is limited by its reliance on passively notified cases of clinically apparent dengue infection. Only a small proportion of all notified cases were dengue IgM positive and the lowest IgM test positivity rates were in 2006, 2007, and 2008. More cases were notified in each of these three years than in any other years except the outbreak years of 1998 and 2009. If the size of the 2006–2008 outbreaks were inflated by the reporting of an unusually large number of non-dengue illnesses, the increase in dengue incidence we have identified may be false. The data reported here are of illness episodes that met the surveillance case definition for dengue, which includes both fever and rash. Measles also presents with fever and rash, and although included in the Expanded Program of Immunization (EPI), there was a large national epidemic in 2009 
. However, the peak of this measles epidemic was in February 2009 and was clearly distinct in timing from the dengue epidemic that peaked in October 2009 
. Rubella is not included in the EPI and may also be confused for dengue. The occurrence of a large rubella outbreak in Hanoi in 2011 suggests that a large rubella outbreak was unlikely to have occurred in the 5 years prior to 2011 
. Chikungunya has not yet been detected in Vietnam. Since dengue often presents as a non-specific febrile illness that is difficult to distinguish clinically from a large range of other illnesses, it is also probable that the reported cases we have used in this analysis significantly under-estimate the true number of dengue infections.
Since anti-dengue virus IgM is not detectable until 3–5 days after infection, and is frequently negative in secondary dengue infections, asays to detect anti-dengue virus IgM antibodies have limited sensitivity (perhaps <20%) when used alone to diagnose dengue infections during the acute illness 
. Consequently a single negative anti-dengue virus IgM ELISA from an acutely ill patient with clinically suspected dengue is not very helpful in excluding dengue infection. There are also limitations to the specificity of a positive dengue IgM ELISA. IgM ELISA's can remain positive for three months or longer after initial dengue infection, therefore febrile illnesses following dengue infection may be incorrectly diagnosed as dengue on the basis of persisting anti-dengue IgM 
. False positive results are also possible as a result of cross-reactivity with anti-bodies to Japanese Encephalitis (JE) following infection or vaccination. However JE in Vietnam is predominantly a low-incidence rural problem and the immunization programme has targeted children in higher risk rural Provinces 
. Therefore it is unlikely that a significant number of dengue IgM positive cases in Hanoi are false positives due to cross-reaction with JE antibodies. Given these limitations, interpretation of single dengue IgM results is difficult, and other diagnostic approaches, such as combined NS1 antigen and IgM assays, should be considered to augment and aid interpretation of clinical surveillance data 
As dengue disease severity is modulated by age, prior dengue exposure history, and the infecting dengue serotype, the incidence of clinically apparent dengue cases is unlikely to have a linear relationship with transmission intensity 
. Therefore data on the age specific incidence of clinical illness cannot be used to directly infer the age specific risk of infection. Seroepidemiological studies are required for this and should be conducted as part of national surveillance programs. Public health authorities in Vietnam have the impression that dengue is an emerging health problem in Hanoi. Although our analysis supports that conclusion, there is a need for population-based seroepidemiology studies that remove any residual confounding by unmeasured demographic factors, and give an unbiased estimate of the age and serotype specific force of infection. The interpretation of such studies will be improved by the recent development of assays that are able to distinguish the original infecting serotype in secondary dengue infections 
. If the current extensive efforts to develop a dengue vaccine are successful, there exists the exciting potential that appropriately targeted and timed vaccination in areas of low and intermittent transmission may be a cost effective intervention to prevent annual outbreaks or even prevent the (re)establishment of dengue. Such interventions will require a thorough understanding of the dynamics of dengue in such areas.