Sex differences in COVID-19 infection and mortality in Hong Kong

Although infection rates were similar between sexes in Hong Kong, males experienced significantly higher COVID-19 mortality risks across all epidemic waves even after adjusting for vaccination status and pre-existing conditions, a pattern consistent with sex disparities observed in other respiratory diseases.

The Gist

Imagine the COVID-19 pandemic in Hong Kong as a massive, multi-year storm that battered the city. This study is like a detailed weather report that asks a very specific question: Did men and women experience this storm differently?

The researchers, who are essentially "storm trackers" at the University of Hong Kong, looked at data from January 2020 to January 2023. They wanted to know if the virus hit men and women with the same force, and if the aftermath was the same for both groups.

Here is the story of their findings, broken down into simple concepts:

1. The Rain Was the Same, But the Damage Was Different

Think of getting infected with the virus as getting caught in the rain. The study found that men and women got "wet" at the same rate. Whether it was a light drizzle (early waves) or a massive downpour (the big Omicron wave in 2022), the number of men and women catching the virus was roughly equal.

However, once they were "wet," the outcome was different. Men were much more likely to get sick enough to end up in the hospital or, tragically, pass away. It's as if the rain soaked both groups equally, but men's "umbrellas" (their bodies' defenses) seemed to break more easily under the pressure.

2. The "Older Age" Factor

The study zoomed in on older adults, particularly those over 45. In almost every wave of the pandemic, older men faced a higher risk of death than older women.

Even when the researchers adjusted for the usual suspects—like if someone had high blood pressure, diabetes, or heart disease—the gap remained. It's like saying, "Even if we give both the man and the woman the same heavy backpack (chronic diseases), the man is still more likely to stumble and fall."

3. The "Pre-Pandemic" Clue

One of the most interesting parts of the study is a historical look-back. The researchers checked death rates from other respiratory diseases (like the flu or pneumonia) between 2000 and 2019, before COVID-19 even existed.

They found that men were already dying from lung and breathing issues at higher rates than women. The ratio of male-to-female deaths for COVID-19 was almost identical to the ratio for these other breathing diseases.

The Analogy: Imagine a car crash. If men are statistically more likely to die in car crashes than women before a specific new type of car is introduced, and then they are still more likely to die in crashes with the new car, it suggests the issue isn't just the new car (the virus). It suggests there are deeper, pre-existing reasons why men are more vulnerable to traffic accidents (respiratory illness) in general.

4. Why the Difference?

The paper suggests a few reasons why men's "umbrellas" might be weaker:

• Biology: Men might have different immune system responses.
• Habits: In Hong Kong, smoking is much more common among men (about 16.7%) than women (3%). Smoking is like rusting the engine of a car; it makes the lungs less able to handle the stress of a virus.
• Underlying Health: Men in Hong Kong have higher rates of certain heart conditions compared to women.

5. The Takeaway

The main message is that COVID-19 didn't invent a new weakness for men; it just exposed an old one.

While the virus infected men and women equally, men were consistently more likely to suffer severe consequences. This wasn't because men didn't get healthcare (Hong Kong has excellent, accessible healthcare for everyone). It was likely a mix of biology, lifestyle choices like smoking, and pre-existing health conditions that made men more fragile when the storm hit.

In short: The virus treated men and women the same at the door, but once inside, the house was much harder for men to survive in. Understanding this helps doctors and public health officials prepare better for the next storm, knowing that men might need extra support to keep their "umbrellas" strong.

Technical Summary

1. Problem Statement

While global data consistently indicates that males experience higher COVID-19 mortality rates than females, the underlying mechanisms remain debated. Hypotheses range from biological factors (e.g., X-chromosome immune gene density) to social determinants (e.g., occupational exposure, healthcare access). However, findings vary by region and development level. In Hong Kong, a jurisdiction with a highly developed, universal healthcare system, the initial spread of SARS-CoV-2 was effectively suppressed for two years before the Omicron subvariants caused a massive surge in mortality in 2022, primarily among the elderly. The study aims to:

• Quantify sex differences in infection, hospitalization, and mortality rates across eight distinct epidemic waves in Hong Kong.
• Determine if the observed male excess mortality persists after adjusting for key confounders (vaccination status, pre-existing chronic conditions, and age).
• Compare the sex ratio of COVID-19 mortality to historical mortality ratios for other respiratory diseases to assess if the disparity is unique to SARS-CoV-2.

2. Methodology

• Data Source: Individual-level patient data for all confirmed SARS-CoV-2 cases in Hong Kong from January 23, 2020, to January 29, 2023. Data included demographics (age, sex), clinical outcomes (discharge/death), comorbidities, and vaccination history.
• Study Period & Waves: The pandemic was segmented into eight waves. Waves 1–4 (pre-Omicron, low transmission) were combined for analysis due to small sample sizes. Waves 5–8 covered the Omicron era, including the massive Wave 5 outbreak in early 2022.
• Metrics Calculated:
  • Incidence rates (cases, hospitalizations, deaths per 100,000 population).
  • Case-Hospitalization Rate (CHR), Case-Fatality Risk (CFR), and Hospital-Fatality Risk (HFR).
  • Note: To ensure comparability across waves, "hospitalized" cases were redefined as those with severe, critical, or fatal outcomes, rather than all admissions, to account for policy changes where mild cases were isolated in hospitals during early waves.
• Statistical Analysis:
  • Descriptive: Wilson score intervals were used to estimate rates and uncertainty intervals, particularly for age groups with low death counts.
  • Regression: Logistic regression models were employed to calculate unadjusted and adjusted Odds Ratios (ORs) for mortality. The adjusted model controlled for age, vaccination status (dose-dependent), and a comprehensive list of comorbidities (diabetes, hypertension, obesity, neoplasm, liver/kidney disease, COPD, cardiovascular conditions, etc.).
  • Historical Comparison: Mortality ratios for respiratory diseases (excluding COVID-19) from 2000–2019 were calculated to establish a baseline sex disparity.
• Software: Analyses were conducted using R version 4.3.3.

3. Key Contributions

• Comprehensive Longitudinal Analysis: The study covers the entire first three years of the pandemic in Hong Kong, capturing the transition from low-transmission phases to the high-mortality Omicron era.
• Robust Adjustment: Unlike many studies that rely on unadjusted data, this research rigorously adjusts for vaccination status and a wide array of pre-existing chronic conditions, isolating the independent effect of sex on mortality.
• Contextual Benchmarking: By comparing COVID-19 sex ratios to pre-pandemic respiratory disease mortality, the study contextualizes whether the male excess is a novel phenomenon or consistent with historical respiratory patterns.

4. Key Results

• Infection Rates: There was no significant difference in SARS-CoV-2 infection rates between males and females across all eight waves.
• Mortality Disparity: Males consistently experienced higher mortality risks than females.
  • Wave 5 Peak: During the Omicron BA.2 surge (Wave 5), the mortality rate for males was 152.1 per 100,000, compared to 86.0 per 100,000 for females.
  • Case-Fatality Risk (CFR): CFRs were significantly higher for males across all waves, particularly in the 80+ age group.
• Adjusted Risk: Even after adjusting for age, vaccination status, and comorbidities, males retained a significantly higher risk of death.
  • Odds Ratios: The adjusted OR for male mortality ranged from 1.44 to 1.51 across waves. In specific waves (e.g., Wave 5 and 7), adjusted ORs ranged from 1.4 to 2.4 depending on the age group.
  • Risk Factors: Older age (80+) and pre-existing kidney disease (OR 2.18 overall; OR 11.54 in early waves) were the strongest predictors of death. Vaccination showed a clear dose-dependent protective effect.
• Historical Comparison: The male-to-female mortality ratio for COVID-19 (approx. 1.7–2.6) was remarkably similar to the ratio observed for other respiratory diseases in Hong Kong between 2000 and 2019.

5. Significance and Conclusion

• Biological vs. Social Drivers: The persistence of the male mortality excess after adjusting for healthcare access (universal in HK), vaccination, and comorbidities suggests that biological factors (e.g., higher ACE2 receptor expression, immune response differences) and behavioral risks (e.g., higher smoking prevalence in HK males: 16.7% vs. 3.0% in females) play a critical role.
• Policy Implications: The findings indicate that the male excess mortality in COVID-19 is not an anomaly but likely reflects shared vulnerabilities with other respiratory diseases. Public health strategies for future pandemics should account for these sex-specific vulnerabilities, particularly regarding smoking cessation and targeted protection for males with chronic kidney or cardiovascular conditions.
• Limitations: The study acknowledges potential biases in case identification due to self-reported rapid antigen tests (RATs) in later waves and the inability to model sex-specific immune responses directly. However, the consistency of the mortality ratio with historical respiratory data strengthens the validity of the core findings.

Conclusion: In Hong Kong, while males and females had similar infection rates, males faced a significantly higher risk of severe disease and death. This disparity persisted even after controlling for known risk factors, aligning with historical patterns of respiratory disease mortality and pointing to intrinsic biological and behavioral sex differences as key drivers.