COVID-19: Understanding What We Know After Five Years

 

COVID-19: Understanding What We Know After Five Years

Introduction

Five years after COVID-19 transformed daily life around the world, our understanding of the disease is far more advanced than it was in early 2020. What began as a mysterious outbreak of pneumonia became a global pandemic that disrupted health systems, economies, education, travel, politics, and social relationships. Millions of people died, billions were infected, and societies were forced to make difficult decisions with incomplete information.

Today, COVID-19 is no longer the same public-health emergency it was during the first years of the pandemic. However, it has not disappeared. SARS-CoV-2, the virus that causes COVID-19, continues to circulate, evolve, and cause hospitalizations and deaths. At the same time, population immunity has increased through vaccination and previous infection, treatments have improved, and health systems have gained considerable experience in managing severe disease. The World Health Organization (WHO) now describes the global risk as substantially lower than during the acute pandemic period, while emphasizing continued surveillance and protection of people at higher risk.

The five years since the pandemic began have therefore produced a more complicated picture than either "COVID-19 is over" or "nothing has changed." We now know much more about how the virus spreads, who is most vulnerable, how vaccines protect people, why variants emerge, and why some people develop long-term symptoms. We have also learned painful lessons about inequalities, misinformation, public-health communication, and global preparedness.

This article examines what science and public health have learned about COVID-19 after five years, what remains uncertain, and what lessons should guide the world in preparing for future pandemics.

From a New Virus to a Continuing Respiratory Disease

SARS-CoV-2 belongs to a large family of coronaviruses. When the virus emerged in late 2019, humans had very little pre-existing immunity to it. This helped the virus spread rapidly across populations. The early pandemic was characterized by uncertainty: researchers were still determining how infectious the virus was, which symptoms were most important, how frequently severe disease occurred, and how transmission could be reduced.

As research expanded, scientists learned that SARS-CoV-2 could spread efficiently from person to person, including from people who had few or no symptoms. Transmission through respiratory particles and aerosols became increasingly important to understanding the pandemic. This helped explain why crowded, poorly ventilated indoor environments could facilitate transmission.

The virus itself also changed. Variants emerged as SARS-CoV-2 accumulated mutations. Some variants gained advantages in transmission or immune evasion. Alpha and Delta were associated with major waves of infection, while Omicron and its descendants became particularly successful at spreading through populations with considerable immunity.

Five years of genomic surveillance have provided scientists with an extraordinary amount of information about viral evolution. By 2025, researchers had analyzed millions of SARS-CoV-2 genome sequences, giving virologists an unusually detailed view of how a virus evolves in real time.

The emergence of variants also taught an important lesson: a respiratory virus does not have to become uniformly "more deadly" to remain a major public-health threat. A variant that spreads much more efficiently can produce a large burden of illness even if the average severity of individual infections is lower. In addition, changes in population immunity, age distribution, vaccination, previous infections, and access to treatment all influence the eventual impact of a variant.

Today, SARS-CoV-2 is best understood as an ongoing respiratory pathogen rather than a temporary event that simply vanished when the emergency ended. WHO has reported that the virus continues to circulate widely, while severe disease has declined because of widespread immunity and improvements in clinical management.

What We Know About Severe COVID-19

One of the clearest scientific lessons from the pandemic is that COVID-19 does not affect everyone equally. For many healthy people, particularly those with immunity from vaccination or previous infection, an infection may be mild or moderate. For others, however, the disease can still be dangerous.

Older adults remain among those at greatest risk of severe illness and death. People with certain underlying medical conditions and people who are immunocompromised can also face substantially higher risks. Pregnant women have also been recognized as a group requiring particular attention because COVID-19 can increase the risk of serious maternal and pregnancy-related complications.

The risk profile has changed since 2020. A person's outcome today is influenced not only by the virus but also by immunity, age, previous infections, vaccination status, underlying conditions, access to healthcare, and available treatments.

This distinction is important because the number of infections alone does not tell us how dangerous a period of COVID-19 transmission is. A wave that produces many infections but relatively few hospitalizations can have a very different public-health impact from a wave that produces fewer infections but more severe disease.

Another major lesson is that reported case numbers became increasingly difficult to interpret as widespread testing declined. During the early pandemic, governments conducted extensive testing and reported large numbers of confirmed infections. Later, many people stopped testing or used tests without reporting their results. As a result, official case counts no longer provide a complete picture of transmission.

For this reason, public-health researchers increasingly rely on multiple forms of surveillance, including hospitalization data, death reporting, wastewater monitoring, laboratory testing, and genomic sequencing.

The True Death Toll Is Larger Than the Reported Number

One of the most difficult questions about COVID-19 is how many people actually died because of the pandemic.

Officially reported COVID-19 deaths provide important information, but they do not capture the entire impact. Some people died without being tested. In some countries, death certification systems were incomplete. In other cases, people died indirectly because hospitals were overwhelmed or routine healthcare was disrupted.

This is why researchers use the concept of excess mortality. Excess mortality compares the number of deaths that occurred during a crisis with the number of deaths that would normally have been expected. It can therefore capture both direct and indirect effects of the pandemic.

WHO explains that excess mortality includes deaths directly caused by COVID-19, deaths indirectly associated with the pandemic, and adjustments for deaths that may have been prevented because of changes such as reduced traffic or reduced transmission of other infectious diseases.

For 2020 and 2021 alone, WHO estimated approximately 14.91 million excess deaths associated with the pandemic, substantially higher than the number of deaths officially reported as directly attributable to COVID-19 during that period.

More recent WHO assessments estimate that the pandemic was associated with about 22.1 million excess deaths between 2020 and 2023, including indirect deaths.

These figures demonstrate why the pandemic cannot be measured simply by counting laboratory-confirmed COVID-19 deaths. The consequences extended beyond the virus itself and affected healthcare systems and societies on a massive scale.

Vaccines Changed the Course of the Pandemic

Perhaps the greatest scientific achievement of the COVID-19 response was the development and deployment of effective vaccines at unprecedented speed.

Before the pandemic, vaccine development commonly took many years. COVID-19 changed that timeline. Scientists benefited from decades of research into coronavirus biology, messenger RNA technology, viral vectors, protein-based vaccines, and immune responses. Once SARS-CoV-2 was identified and its genetic sequence became available, researchers could rapidly begin developing vaccine candidates.

The first vaccines were introduced in late 2020 and early 2021. Their most important contribution was not eliminating every infection. Instead, they substantially reduced the risk of severe disease and death.

Five years of accumulated safety data have strengthened the evidence supporting the safety of currently available COVID-19 vaccines. WHO reports that more than 13 billion doses had been administered globally and that serious adverse events remain rare relative to the enormous number of doses given.

Vaccines are not perfect. Protection against infection can decline, particularly as immunity wanes and new variants emerge. However, protection against severe outcomes has remained the central benefit.

This distinction became especially important during the Omicron era. Highly transmissible variants caused large numbers of infections, including among vaccinated people. Some interpreted these breakthrough infections as evidence that vaccines had failed. In reality, preventing every infection was never the only or most important measure of vaccine effectiveness. A vaccine that converts a potentially life-threatening illness into a mild infection can still provide enormous public-health value.

By 2026, WHO's approach has increasingly focused on routine, risk-based vaccination rather than mass vaccination of entire populations. Its current recommendations emphasize people at highest risk of severe disease, including older adults and certain people with significant underlying conditions.

Treatments Have Improved Too

Vaccination is only one part of the progress made since 2020. Doctors and researchers also learned how to treat severe COVID-19 more effectively.

Early in the pandemic, clinicians had limited experience with the disease. Hospitals developed protocols for oxygen therapy, respiratory support, anticoagulation, and the use of anti-inflammatory medicines. Clinical trials eventually identified treatments that could reduce complications and deaths among appropriate patients.

The experience also taught healthcare professionals what not to do. Some treatments received widespread attention before reliable evidence showed whether they worked. Large clinical trials eventually helped separate promising approaches from ineffective ones.

This illustrates one of the most important lessons of the pandemic: in a rapidly changing health crisis, medical decisions need strong evidence. Randomized clinical trials and carefully collected real-world data are essential, particularly when public pressure encourages governments and physicians to act before sufficient evidence is available.

Today, clinicians have considerably more knowledge about which patients are at high risk, when antiviral treatment may be useful, how to manage severe respiratory illness, and how to monitor complications.

Long COVID: The Pandemic's Continuing Legacy

One of the most important discoveries of the pandemic has been the recognition that COVID-19 can have consequences that persist long after the initial infection.

Post-COVID-19 condition, commonly called long COVID, refers to a range of symptoms and health problems that continue or develop after the acute infection. Symptoms can include fatigue, breathlessness, cognitive difficulties, sleep problems, and muscle or joint pain.

The exact biological mechanisms are still being investigated. Researchers have proposed several possibilities, including persistent immune activation, changes involving blood vessels, nervous-system effects, viral remnants, and disruptions to normal immune regulation. It is possible that long COVID represents several overlapping conditions rather than one single disease mechanism.

Estimates of prevalence vary because studies use different definitions and populations. WHO's more recent assessments suggest that approximately 6% of symptomatic infections result in post-COVID-19 condition, although the risk appears to have declined compared with earlier stages of the pandemic.

Importantly, long COVID is not limited to people who were hospitalized. Because mild infections are so common, a large proportion of long-COVID cases can occur after relatively mild initial illness. WHO noted in its 2024 epidemiological assessment that more than 90% of post-COVID cases arose after mild infections, largely because mild infections account for such a large share of total infections.

Vaccination also appears to reduce the risk of developing long COVID, adding another reason to protect people against infection and severe disease.

Long COVID remains an important research and healthcare challenge. There is no single diagnostic test that identifies every case, and treatment often focuses on managing symptoms, rehabilitation, and supporting patients while researchers continue to investigate underlying mechanisms.

What We Learned About Immunity

COVID-19 also transformed scientific understanding of human immunity.

Researchers learned that immunity is complex and changes over time. Infection and vaccination can generate antibodies and cellular immune responses, but these responses do not remain constant indefinitely. Protection against infection can decline, while protection against severe disease can persist longer.

Researchers also learned that the immune system responds differently to different variants. The Omicron family demonstrated how a virus can accumulate mutations that allow it to partially escape antibodies generated by earlier infections or vaccines.

At the same time, studies of immune responses helped reveal why vaccination remains valuable even when infections occur. The immune system has multiple layers of defense. Antibodies can help prevent infection, while other immune responses can help limit disease after infection has occurred.

The pandemic therefore challenged the simplistic idea that immunity is either "present" or "absent." Immunity is better understood as a changing spectrum influenced by vaccination, previous infections, time, age, health status, and the characteristics of the virus.

This knowledge will be valuable far beyond COVID-19. Researchers gained new insights into how immune memory develops, how respiratory viruses interact with the immune system, and why protection can differ between preventing infection and preventing severe illness.

The Importance of Ventilation and Airborne Transmission

Another major lesson concerns how respiratory viruses spread.

Early public-health messaging placed substantial emphasis on hand hygiene and surface disinfection. These measures can be useful, but scientists increasingly recognized that SARS-CoV-2 could spread efficiently through respiratory particles and aerosols in indoor environments.

This changed how experts think about infection prevention. Ventilation, air filtration, outdoor activities, and reducing exposure in crowded indoor environments became important tools.

The lesson extends beyond COVID-19. Better ventilation can potentially reduce transmission of influenza and other respiratory infections as well.

The pandemic demonstrated that buildings are part of public health. Schools, offices, hospitals, restaurants, factories, and public transportation systems all have environments that can influence respiratory-virus transmission.

Misinformation Became a Second Pandemic

Scientific knowledge was not the only thing that evolved during COVID-19. Information itself became a major public-health issue.

The internet allowed scientific findings to spread rapidly, but it also allowed rumors, conspiracy theories, false medical claims, and misleading statistics to circulate at extraordinary speed.

Public-health authorities sometimes struggled to communicate uncertainty. Scientific recommendations naturally changed as evidence improved, but members of the public could interpret changing guidance as evidence that experts did not know what they were doing.

For example, recommendations regarding masks, transmission, vaccination, and treatment evolved as researchers learned more. Changing advice is not necessarily evidence of failure; in science, conclusions should change when evidence changes. However, communicating that process clearly proved difficult.

The pandemic therefore showed that effective public health requires more than scientific expertise. Authorities must explain what is known, what is uncertain, why recommendations may change, and how confident experts are in different conclusions.

Nature's review of the pandemic's lessons emphasized that politicization and misinformation weakened trust in public-health institutions and increased vaccine hesitancy. It argued that future preparedness must include stronger communication and cooperation between public-health experts and social scientists who study misinformation.

Inequality Was a Central Feature of the Pandemic

COVID-19 also revealed enormous inequalities between and within countries.

The pandemic did not affect every population equally. Wealthier countries generally had earlier access to vaccines, testing capacity, hospital resources, and advanced treatments. Many lower-income countries faced shortages of vaccines, oxygen, healthcare workers, and diagnostic capacity.

Even within wealthy countries, social and economic circumstances influenced risk. People who could work from home often had greater ability to avoid exposure than people whose jobs required constant contact with customers, patients, or coworkers.

Housing conditions also mattered. A person living alone in a spacious home had very different opportunities to isolate from someone living in a crowded household.

The unequal distribution of vaccines was particularly significant. High-income countries initially secured much greater access to vaccines, while many low- and middle-income countries waited longer.

This created an important lesson for future pandemics: global health security cannot depend entirely on individual countries competing for scarce medical supplies. A pandemic anywhere can eventually affect people everywhere, making equitable access not only a moral issue but also a practical one.

The Origin Question Remains Important

Five years later, the origin of SARS-CoV-2 remains a subject of scientific and political debate.

Scientists have investigated several hypotheses, including zoonotic transmission from animals to humans. Research has identified evidence relevant to animal exposure and market environments, and studies have highlighted raccoon dogs and other susceptible animals as possible contributors to the early transmission chain. However, important questions remain unresolved.

The scientific importance of understanding origins goes beyond assigning blame. Determining how a pandemic virus entered human populations can help researchers identify risk factors and prevent future outbreaks.

At the same time, discussions about origins should distinguish between evidence, hypotheses, and political claims. Scientific uncertainty is not evidence that any particular explanation is automatically correct.

The broader lesson is that countries need transparent outbreak investigations and stronger systems for monitoring viruses at the human-animal interface.

Pandemic Preparedness Is Still Incomplete

Perhaps the most uncomfortable lesson is that the world is better prepared in some ways but not necessarily prepared enough.

The pandemic accelerated vaccine technology, genomic sequencing, clinical-trial infrastructure, wastewater surveillance, and international scientific cooperation. Researchers now have much more experience responding to a rapidly evolving respiratory virus.

Yet the political and financial commitment to preparedness can weaken once the immediate crisis fades.

Nature warned in 2025 that countries risk losing the urgency that followed the early pandemic and argued that the world should not assume another pandemic is far in the future.

Preparedness requires sustained investment in laboratories, hospitals, public-health agencies, supply chains, surveillance systems, healthcare workers, vaccine manufacturing, and emergency communication.

It also requires international cooperation. Viruses do not respect national borders. A country with excellent surveillance can still be threatened by an outbreak elsewhere if information is delayed or medical resources are distributed unfairly.

Future preparedness should therefore focus on building systems that work during ordinary times, rather than creating temporary structures only after a crisis begins.

What We Still Do Not Know

Despite enormous scientific progress, important questions remain.

Researchers still do not fully understand why some people develop long COVID while others recover quickly. The precise mechanisms driving many persistent symptoms remain under investigation.

Scientists also cannot predict exactly how SARS-CoV-2 will evolve. Genomic surveillance can identify new variants, but predicting which mutations will dominate in the future remains difficult.

The duration and quality of immunity also remain important areas of study. People differ considerably in their responses to vaccination and infection, and immune protection changes over time.

Another uncertainty concerns the long-term relationship between SARS-CoV-2 and human populations. The virus is likely to continue evolving, but the exact pattern of future waves, variants, and seasonal activity cannot be known with certainty.

Finally, many countries still have significant gaps in health data. WHO has emphasized that reduced testing, genomic sequencing, and mortality reporting make it harder to accurately measure the continuing global burden of COVID-19.

This is a critical lesson: when surveillance disappears, uncertainty increases. A disease does not become harmless simply because fewer people are measuring it.

Five Years of Lessons for the Future

Looking back, several lessons stand out.

First, early action matters. Once a respiratory virus begins spreading internationally, delays can have enormous consequences.

Second, science must be flexible. Recommendations should change when evidence changes, and authorities should communicate why.

Third, vaccines are powerful but not perfect. Their most important benefit is protection against severe disease and death, while additional strategies remain necessary.

Fourth, health systems must be resilient. Hospitals need the capacity to expand during emergencies without completely abandoning routine care.

Fifth, surveillance is essential. Testing, sequencing, wastewater monitoring, mortality statistics, and hospital data allow governments to detect changes before they become overwhelming.

Sixth, communication is a public-health intervention. Accurate information must be communicated clearly and consistently, while misinformation must be addressed without unnecessarily deepening social divisions.

Seventh, global inequality creates global vulnerability. A pandemic response cannot be considered successful if lifesaving technologies reach some countries years before others.

Finally, preparedness must continue after the emergency ends. The greatest danger may come when societies become so tired of a crisis that they stop investing in preventing the next one.

Conclusion

Five years after the beginning of the COVID-19 pandemic, the world knows vastly more about SARS-CoV-2 than it did in 2020. Scientists understand its transmission, evolution, immune interactions, clinical effects, and prevention strategies far better. Vaccines have saved lives, treatments have improved, and health systems have gained valuable experience.

Yet the story is not finished.

COVID-19 continues to circulate, and vulnerable people remain at risk of hospitalization and death. Long COVID affects a significant minority of infected people. Surveillance has weakened in many places, making it harder to measure the virus accurately. And the broader social consequences of the pandemic—including disrupted education, economic losses, political polarization, mental-health challenges, and declining trust in institutions—will remain subjects of research for years.

The most important lesson is therefore neither that COVID-19 was harmless nor that society must remain permanently in emergency mode. The evidence points to something more balanced: COVID-19 has become a manageable but continuing public-health threat, and our ability to manage it depends on maintaining the scientific, medical, and institutional knowledge gained during the pandemic.

The pandemic also demonstrated that preparedness is not simply about developing a vaccine after a crisis begins. It is about maintaining strong health systems, trustworthy institutions, reliable information, equitable access to medical technologies, and international cooperation before the next crisis arrives.

Five years of COVID-19 have taught humanity that scientific progress can move extraordinarily quickly when resources and cooperation are available. They have also shown how vulnerable societies become when preparation, communication, and solidarity fall behind.

The goal for the future should not be to forget COVID-19. It should be to learn from it. The next pandemic may involve a different virus and present different challenges. But the lessons of COVID-19 provide a foundation: detect threats early, follow evidence, communicate honestly, protect vulnerable people, share resources fairly, and invest in preparedness even when the crisis is no longer dominating the headlines.

In that sense, understanding what we know after five years is not merely an exercise in looking backward. It is preparation for what comes next.