Surveillance and annual vaccination are needed for at-risk populations in tropical countries.

We used a regression model to examine the impact of influenza on death rates in tropical Singapore for the period 1996–2003. Influenza A (H3N2) was the predominant circulating influenza virus subtype, with consistently significant and robust effect on mortality rates. Influenza was associated with an annual death rate from all causes, from underlying pneumonia and influenza, and from underlying circulatory and respiratory conditions of 14.8 (95% confidence interval 9.8–19.8), 2.9 (1.0–5.0), and 11.9 (8.3–15.7) per 100,000 person-years, respectively. These results are comparable with observations in the United States and subtropical Hong Kong. An estimated 6.5% of underlying pneumonia and influenza deaths were attributable to influenza. The proportion of influenza-associated deaths was 11.3 times higher in persons age >65 years than in the general population. Our findings support the need for influenza surveillance and annual influenza vaccination for at-risk populations in tropical countries.

Influenza virus infections cause excess illness and deaths in temperate countries. In the Northern and Southern Hemispheres, influenza epidemics occur nearly every winter, leading to an increase in hospitalizations and deaths. The World Health Organization (WHO) estimated that these annual epidemics result in 3 to 5 million cases of severe illness and 250,000–500,000 deaths each year around the world (

However, little is known about the impact of influenza on death rates in tropical regions, where the effect of influenza is thought to be less (

In tropical Singapore, influenza viruses circulate year round, with a bimodal increase in influenza incidence observed in April–July and November–January (

Any assessment of the true impact of influenza in the tropics must account for the more diffused seasonal pattern of influenza in the tropics and the cocirculation of other respiratory viruses. Respiratory syncytial virus (RSV) is also associated with excess deaths (

Influenza virus surveillance is carried out throughout the year and has been instituted in Singapore since 1973. We obtained monthly data on influenza A and B viruses and RSV from the WHO-designated National Influenza Centre in Singapore from January 1996 to December 2003. Specimens tested for influenza and RSV were obtained from pediatric inpatients at KK Women's and Children's Hospital, patients from Singapore General Hospital and other public-sector hospitals, as well as from adult outpatients with influenzalike symptoms treated at sentinel primary health clinics. Specimens were tested either with informed consent from patients for diagnostic purposes or as part of epidemiologic surveillance provided for by the Infectious Diseases Act.

RSV was detected by immunofluorescence tests and virus isolation. Influenza viruses were identified by direct antigen detection with immunofluorescence techniques, serologic tests with complement fixation, and virus isolation. To isolate influenza viruses, respiratory specimens were added to primary cynomolgus monkey kidney tissue cultures, which were rolled at 33°C and observed daily for cytopathic effects. If no effect was observed, the HeLa tubes were passaged blind at weekly intervals, and monkey kidney tissue cultures were tested for hemadsorption with guinea pig erythrocytes. Specimens were discarded after 4 weeks if negative. Influenza virus isolates were subsequently confirmed by immunofluorescence and typed by hemagglutination-inhibition tests using strain-specific antisera provided by the WHO Collaborating Centre for Influenza at the Centers for Disease Control and Prevention, Atlanta, Georgia, USA.

The National Influenza Center provided aggregated data for this study, i.e., monthly numbers of total respiratory specimens tested for influenza virus, positive influenza test results, and influenza virus isolates by subtype, as well as monthly RSV data. As the study spanned 8 years, we anticipated that positive results could be affected by changes in the number of tests performed. Therefore, we opted to use the monthly proportion of positive test results for a specific virus (with the respective monthly number of specimens tested for the specific virus as the denominator) as our indicator variable for virus activity, instead of monthly positive counts.

National mortality data were obtained from the Registry of Births and Deaths. Under the Registration of Births and Deaths Act, all deaths occurring within Singapore and its territorial waters are required to be registered within 3 days of the occurrence. Each death was categorized according to the International Classification of Diseases, 9th Revision (ICD-9) codes. In this study, death records were aggregated according to month of death from January 1996 through December 2003. Three death outcomes were analyzed: underlying pneumonia and influenza (P&I) deaths (ICD-9: 480–487), underlying circulatory and respiratory (C&R) deaths (ICD-9: 390–519), and all-cause deaths (ICD-9: 000–999).

We first applied 6 negative binomial regression models (

Apart from accounting for possible overdispersion of the data in the models, the models also adjusted for potential confounding factors, including the number of days in each month, linear and squared term of time trend, seasonality (3–4 pairs of sine and cosine terms, allowing for 3 to 4 cycles per year to capture the main seasonal variations per year), temperature, and relative humidity. Linear and squared terms of time trend were included to capture secular trends, including population growth, changes in completeness of ICD coding, and changes in diagnostic methods. For each model, residuals were examined for discernible patterns and autocorrelation by means of residual plots and partial autocorrelation function plots. Since the unit of analysis was the calendar month, the lag effects of influenza and other covariates were not necessarily taken into account.

We estimated the influenza-associated mortality fraction by dividing the number of excess deaths (the difference between observed and expected deaths) by the number of observed deaths, when the proportion of positive influenza results was set to 0 in model 6. The 95% CI for each estimated fraction was obtained by using the bootstrap resampling method with 1,000 bootstrap resamples (

From January 1996 to December 2003, 57,060 specimens were tested for influenza virus, and 51,370 were tested for RSV. The volume of tests performed was noticeably lower in the first 2 years and in the last year of the study (

Year | Influenza virus | |||||||
---|---|---|---|---|---|---|---|---|

Influenza type* | Influenza A subtype† | RSV | ||||||

No. specimens tested | Influenza A-positive test results (%) | Influenza B-positive test results (%) | No. specimens tested | A (H1N1)-positive isolates (%) | A (H3N2)-positive isolates (%) | No. specimens tested | Total positive test results (%) | |

1996 | 5,140 | 132 (2.6) | 47 (0.9) | 924 | 1 (0.1) | 15 (1.6) | 4,249 | 868 (20.4) |

1997 | 5,255 | 208 (4.0) | 39 (0.7) | 1,041 | 9 (0.9) | 17 (1.6) | 4,441 | 902 (20.3) |

1998 | 8,934 | 817 (9.1) | 120 (1.3) | 941 | 3 (0.3) | 40 (4.3) | 7,573 | 1,683 (22.2) |

1999 | 7,548 | 714 (9.5) | 74 (1.0) | 1,001 | 1 (0.1) | 99 (9.9) | 6,915 | 1,004 (14.5) |

2000 | 7,716 | 397 (5.1) | 122 (1.6) | 974 | 34 (3.5) | 61 (6.3) | 7,094 | 1,425 (20.1) |

2001 | 8,171 | 300 (3.7) | 76 (0.9) | 1,023 | 33 (3.2) | 44 (4.3) | 7,445 | 1,415 (19.0) |

2002 | 8,317 | 274 (3.3) | 34 (0.4) | 897 | 3 (0.3) | 58 (6.5) | 7,840 | 1,128 (14.4) |

2003 | 5,979 | 454 (7.9) | 21 (0.4) | 1,130 | 6 (0.5) | 121 (10.7) | 5,813 | 678 (11.7) |

Mean | 7,133 | 412 (5.8) | 67 (0.9) | 991 | 11 (1.1) | 57 (5.7) | 6,421 | 1,138 (17.8) |

*Respiratory specimens were tested for influenza by virus isolation, direct antigen detection, and serologic tests. †Influenza A isolates obtained from virus isolation were subtyped by using strain-specific antisera from the Centers for Disease Control and Prevention, Atlanta, GA, USA.

During the 8-year period, an annual mean of 15,616 deaths (range 15,301–16,024) occurred in Singapore. An average of 1,798 (range 1,545–2,340) underlying P&I deaths and 8,237 (range 7,833–8,715) underlying C&R deaths occurred each year (

Year | No. underlying P&I deaths (ICD-9: 480–487) | No. underlying C&R deaths (ICD-9: 390–519) | All-cause deaths (ICD-9: 000–999) |
---|---|---|---|

1996 | 1,690 | 8,420 | 15,569 |

1997 | 1,551 | 8,065 | 15,301 |

1998 | 1,781 | 8,286 | 15,649 |

1999 | 1,640 | 8,169 | 15,513 |

2000 | 1,795 | 8,253 | 15,691 |

2001 | 1,545 | 7,833 | 15,368 |

2002 | 2,077 | 8,158 | 15,811 |

2003 | 2,340 | 8,715 | 16,024 |

*P&I, pneumonia and influenza; C&R, circulatory and respiratory; ICD-9, International Classification of Diseases, 9th Revision.

The

emporal trends in the positivity of specific respiratory viruses (influenza A, influenza B, and respiratory syncytial virus [RSV]) and the number of all-cause deaths (A), underlying pneumonia and influenza (P&I) deaths (B), and underlying circulatory and respiratory (C&R) deaths (C), January 1996–December 2003; +ve %, percent positive.

We tested the Spearman rank correlations between influenza and RSV, and meteorologic variables. Influenza A positivity (Spearman correlation [r] = 0.25) was weakly correlated with relative humidity. However, temperature (r = –0.71) was highly correlated with relative humidity. The influenza A (H3N2) subtype had a high correlation with influenza A (r = 0.75) (data not shown).

The relationship between deaths and each respiratory virus (influenza A, influenza B, and RSV) was examined by using a stepwise sequential approach (

Mortality outcome/risk factor | Adjusted risk ratio (95% CI), p value | |||||
---|---|---|---|---|---|---|

Model 1§ | Model 2 | Model 3 | Model 4 | Model 5 | Model 6 | |

All-cause deaths | ||||||

Influenza A | 1.05 (1.04–1.06), 0.000 | – | – | 1.05 (1.04–1.06), 0.000 | 1.05 (1.04–1.06), 0.000 | 1.05 (1.04–1.06), 0.000 |

Influenza B | – | 1.01 (1.00–1.02), 0.173 | – | 1.01 (1.01–1.02), 0.001 | – | 1.01 (1.01–1.02), 0.001 |

RSV | – | – | 1.00 (0.99–1.00), 0.810 | – | 1.00 (1.00–1.01), 0.254 | 1.00 (1.00–1.01), 0.159 |

Underlying P&I deaths | ||||||

Influenza A | 1.12 (1.08–1.16), 0.000 | – | – | 1.12 (1.08–1.16), 0.000 | 1.13 (1.09–1.17), 0.000 | 1.13 (1.09–1.17), 0.000 |

Influenza B | – | 0.99 (0.96–1.02), 0.389 | – | 1.00 (0.94–1.03), 0.994 | – | 1.00 (0.98–1.03), 0.872 |

RSV | – | – | 1.01 (0.99–1.02), 0.342 | – | 1.03 (1.00–1.02), 0.022 | 1.01 (1.00–1.02), 0.021 |

Underlying C&R deaths | ||||||

Influenza A | 1.08 (1.06–1.10), 0.000 | – | – | 1.08 (1.07–1.10), 0.000 | 1.08 (1.06–1.11), 0.000 | 1.09 (1.07–1.11), 0.000 |

Influenza B | – | 1.01 (0.99–1.02), 0.360 | – | 1.02 (1.01–1.03), 0.004 | – | 1.02 (1.01–1.03), 0.002 |

RSV | – | – | 1.00 (0.99–1.01), 0.686 | – | 1.01 (1.00–1.01), 0.025 | 1.01 (1.00–1.01), 0.011 |

*Risk ratio estimates (95% confidence intervals) of each death category were adjusted for number of days in each month, linear and squared time trends, seasonal patterns, temperature and relative humidity; –, risk factor was not included in model. †Each 1% change was used for influenza B because of the small range of positive influenza B test results. ‡CI, confidence interval; RSV, respiratory syncytial virus; P&I, pneumonia and influenza; C&R, circulatory and respiratory. §Negative binomial regression models. Model 1, death outcome = influenza A + confounders; model 2, death outcome = influenza B (FluB) + confounders; model 3, death outcome = RSV + confounders; model 4, death outcome = model 1 + FluB; model 5, death outcome = model 1 +RSV; model 6, death outcome = model 4 + RSV.

Influenza A had significant and robust effects on monthly all-cause deaths (RR 1.05 for each 10% change in positive test results, without adjusting for influenza B virus, RSV, and other potential confounding factors; vs. RR 1.05, after adjusting for influenza B, RSV, and other confounding factors), underlying P&I (RR 1.12 vs. RR 1.13), and underlying C&R (1.08 vs. 1.09) deaths.

In

Model 6 mortality outcome | Adjusted risk ratio (95% CI), p value† | |||
---|---|---|---|---|

Influenza A (H1N1) | Influenza A (H3N2) | Influenza B | RSV | |

All-cause deaths | 1.00 (0.96–1.04), 0.928 | – | 1.01 (1.00–1.02), 0.178 | 1.00 (0.97–1.00), 0.824 |

– | 1.04 (1.02–1.05), 0.000 | 1.01 (1.00–1.02), 0.008 | 1.00 (1.00–1.01), 0.484 | |

Underlying P&I deaths | 1.00 (0.88–1.13), 0.993 | – | 0.99 (0.96–1.02), 0.409 | 1.01 (0.99–1.02), 0.369 |

– | 1.08 (1.04–1.12), 0.000 | 1.00 (0.97–1.03), 0.878 | 1.01 (1.00–1.02), 0.099 | |

Underlying C&R deaths | 1.01 (0.95–1.08), 0.771 | – | 1.01 (0.99–1.02), 0.343 | 1.00 (0.99–1.01), 0.626 |

– | 1.05 (1.04–1.07), 0.000 | 1.01 (1.00–1.03), 0.037 | 1.00 (1.00–1.01), 0.166 |

*CI, confidence interval; RSV, respiratory syncytial virus; P&I, pneumonia and influenza; C&R, circulatory and respiratory. †Risk ratio estimates (95% confidence intervals) of each death category were adjusted for number of days in each month, linear and squared time trends, seasonal patterns, temperature, and relative humidity; –, risk factor was not included in the model.

Influenza B also had a significant effect on underlying C&R deaths (RR 1.01 for each 1% change in positive test results, 95% CI 1.00–1.03, p = 0.037) and all-cause deaths (1.01, 1.00–1.02, p = 0.008), but not on underlying P&I deaths (p = 0.878). RSV had no observable impact on all 3 death categories analyzed (RR range 1.00–1.01 for each 10% change in positive test results, p>0.099) (

Next, we used the full model to quantify the excess deaths attributable to influenza throughout the year. For deaths from all causes, we estimated an annual mean of 588 influenza-associated deaths (

Mortality outcome/age group (y) | Deaths (%) associated with influenza (95% CI)* | No. excess deaths per year (95% CI) | Excess mortality rate/100,000 person-years (95% CI) |
---|---|---|---|

All-cause deaths | |||

All ages | 3.8 (2.5–5.0) | 588 (396–782) | 14.8 (9.8–19.8) |

>65 | 4.2 (2.7–5.6) | 421 (273–571) | 167.8 (107.0–229.5) |

20–64 | 2.3 (0.9–3.7) | 114 (42–186) | 4.2 (1.6–6.8) |

Underlying pneumonia and influenza deaths | |||

All ages | 6.5 (2.2–10.5) | 116 (40–196) | 2.9 (1.0–5.0) |

>65 | 7.7 (3.5–11.7) | 118 (50–189) | 46.9 (20.3–74.6) |

20–64 | 9.6 (3.0–15.7) | 23 (7–39) | 0.8 (0.2–1.4) |

Underlying circulatory and respiratory deaths | |||

All ages | 5.8 (4.0–7.5) | 475 (324–629) | 11.9 (8.3–15.7) |

>65 | 6.2 (4.4–8.1) | 390 (270–512) | 155.4 (108.8–203.0) |

20–64 | 4.6 (2.5–6.7) | 88 (47–131) | 3.2 (1.7–4.8) |

*CI, confidence interval.

We observed that the proportion of influenza-associated deaths was higher among the elderly. The annual influenza-associated proportion of deaths from all causes was 11.3 times higher in persons age >65 years (167.8/100,000 person-years) than in the general population (14.8/100,000). For influenza-associated underlying P&I deaths, the annual death rate in those >65 years (46.9/100,000) was 16.2 times higher than those in the general population (2.9/100,000) (

Author | Country | Statistical method | Influenza-associated mortality rate/100,000 person-years | ||
---|---|---|---|---|---|

All-cause | Underlying pneumonia and influenza deaths | Underlying circulatory and respiratory deaths | |||

Chow et al. | Singapore | Negative binomial regression model was used to estimate mortality outcomes. The model was developed by using monthly number of deaths and monthly proportion of positive influenza test results. Linear and nonlinear time trends, 3–4 pairs of seasonality variables, monthly mean temperature and relative humidity, and monthly proportion of positive respiratory syncytial virus (RSV) test results were included as covariates in the model. | All ages: 14.8 | All ages: 2.9 | All ages: 11.9 |

>65 y: 167.8 | >65 y: 46.9 | >65 y: 155.4 | |||

Wong et al. ( | Hong Kong | Poisson regression model was used to estimate mortality outcomes. The model was developed by using weekly number of deaths and weekly proportion of positive influenza test results. Dummy variables for each year, 2 pairs of seasonality variables, weekly mean temperature and relative humidity, and weekly proportion of positive RSV test results were included as covariates in the model. | All ages: 16.4 | All ages: 4.1 | All ages: 12.4 |

>65 y: 136.1 | >65 y: 39.3 | >65 y: 102.0 | |||

Thompson et al. ( | United States | Age-specific Poisson regression models were used to estimate mortality outcomes. Each model was developed by using weekly number of deaths for the specific age group and weekly proportion of positive influenza test results. Age-specific population size, linear and nonlinear time trends, 1 pair of seasonality variables, and weekly proportion of positive RSV test results were included as covariates in each model. | All ages: 19.6 | All ages: 3.1 | All ages: 13.8 |

>65 y: 132.5 | >65 y: 22.1 | >65 y: 98.3 |

To our knowledge, our findings are the first to demonstrate that influenza activity is associated with excess deaths in a tropical country. Our estimates of annual influenza-associated all-cause deaths, underlying P&I deaths, and underlying C&R deaths in Singapore were 14.8, 2.9, and 11.9 per 100,000 person-years, respectively. This finding would translate to an estimated 588 deaths (3.8% of total deaths) due to influenza annually, which is comparable to the proportion of deaths observed in subtropical Hong Kong (

Our estimate of 46.9 underlying P&I deaths per 100,000 persons age >65 years each year is lower than the estimate of a local study (

In Singapore, we observed that the influenza-associated proportion of deaths was highest in persons >65 years. Again, this finding is consistent with those in the United States where 90% of influenza-associated deaths occurred among the elderly (

In fact, our estimates for influenza-associated deaths in persons age >65 years were consistently higher than those in Hong Kong and United States, for all 3 mortality outcomes. A possible reason could be the use of influenza vaccines among vulnerable elderly is higher in the United States and Hong Kong than in Singapore. Influenza vaccination for all persons age >65 years is a well-established recommendation of the Advisory Committee on Immunization Practice (ACIP) in the United States (

Annual influenza vaccination for persons age >65 years has been recommended since September 2003 in Singapore by the National Expert Committee on Immunization. Influenza vaccine efficacy for preventing death among people >65 years was estimated to be 68% (

With regard to influenza subtypes, we note that most seasons in the United States were dominated by influenza A (H3N2) virus (

One limitation of our study may have been that the effect of RSV could have been obscured when we analyzed data on all ages. This virus is known to predominately affect children <2 years of age (

Studies suggest that global interhemispheric circulation of epidemics follows an irregular pathway with recurrent changes in the leading hemisphere (

Our findings have a few policy implications. First, they support the recent recommendation by the National Expert Committee on Immunization on annual influenza vaccination for elderly Singaporeans and for persons at high risk of having complications from influenza. Second, the finding that influenza infections account for substantial disease supports our continued investment in strengthening influenza surveillance in our country. Finally, the study provides justification for stockpiling antiviral drugs in our national influenza pandemic preparedness plan. An influenza pandemic can be expected to result in far higher attack and death rates (

In 2003, a new infectious disease, severe acute respiratory syndrome (SARS), emerged, which caused 238 cases and 33 deaths in Singapore (

Our study is the first to show unequivocally that influenza has a significant impact on proportion of deaths in a tropical country like Singapore. The estimated excess deaths, while less than that observed in subtropical and temperate countries, still constitutes a substantial problem. As influenza-associated deaths are largely preventable through vaccination and the judicious use of antiviral drugs, our findings can influence the public health management of this disease.

We developed 6 negative binomial regression models to examine the relationships between proportion of deaths and the respiratory viruses, namely, influenza A virus, influenza B virus, and respiratory syncytial virus (RSV) (

Monthly number of deaths = monthly proportion of influenza A + number of days in each month + linear time trend + squared time trend + 3–4 pairs of seasonality variables + monthly mean temperature + monthly mean relative humidity

Monthly number of deaths = monthly proportion of influenza B + number of days in each month + linear time trend + squared time trend + 3–4 pairs of seasonality variables + monthly mean temperature + monthly mean relative humidity

Monthly number of deaths = monthly proportion of RSV + number of days in each month + linear time trend + squared time trend + 3–4 pairs of seasonality variables + monthly mean temperature + monthly mean relative humidity

Monthly number of deaths = monthly proportion of influenza A + monthly proportion of influenza B + number of days in each month + linear time trend + squared time trend + 3–4 pairs of seasonality variables + monthly mean temperature + monthly mean relative humidity

Monthly number of deaths = monthly proportion of Influenza A + monthly proportion of RSV + number of days in each month + linear time trend + squared time trend + 3–4 pairs of seasonality variables + monthly mean temperature + monthly mean relative humidity

Monthly number of deaths = monthly proportion of influenza A + monthly proportion of influenza B + monthly proportion of RSV + number of days in each month + linear time trend + squared time trend + 3–4 pairs of seasonality variables + monthly mean temperature + monthly mean relative humidity

Dr Chow is a public health physician and currently oversees the Communicable Diseases Surveillance branch at Singapore's Ministry of Health. Her research interests include infectious disease epidemiology and public health surveillance.