SARS-CoV-2 continues to circulate year-round, though its speed of spread has changed dramatically since the early pandemic. As of early 2025, global case counts were declining, with the WHO reporting a 16% decrease in new cases over a recent four-week period. But the virus hasn’t settled into a quiet pattern. It surges predictably twice a year, spreads faster than it once did thanks to more transmissible variants, and moves through communities in uneven, clustered bursts rather than a steady wave.
How Fast the Virus Spreads Now
During the 28-day period from January 6 to February 2, 2025, the WHO recorded over 147,000 new confirmed cases globally, a 16% drop compared to the previous month. Weekly test positivity fell from 7.3% to 5.0% over that same stretch. These numbers suggest a post-winter cooldown in many parts of the world. Deaths, however, rose 28% over the same window, a reminder that case counts and severe outcomes don’t always move in sync, especially when testing is less widespread than it used to be.
Official case counts understate actual infections significantly. Most people no longer test, and many infections cause mild or no symptoms. Wastewater surveillance, which detects viral fragments shed by infected people regardless of whether they seek testing, picks up community spread earlier and more completely than clinical data. The CDC’s National Wastewater Surveillance System tracks these trends across the U.S. and consistently shows viral activity even during periods when reported case numbers are low.
Why Newer Variants Spread Faster
The original strain of SARS-CoV-2 that emerged in Wuhan had a basic reproduction number (R0) of roughly 2 to 3, meaning each infected person spread it to about two or three others in a population with no immunity. The Delta variant doubled that, with an R0 estimated between 3.2 and 8. Then Omicron pushed even further, with an R0 of approximately 8.2, making it about 3.8 times more transmissible than Delta. Some Omicron sublineages went further still: the XE recombinant, sometimes called “stealth Omicron,” was estimated to be ten times more infectious than the BA.2 sublineage.
Transmissibility isn’t just about how easily the virus latches onto cells. It’s also about timing. Omicron’s BA.1 subvariant has a median incubation period of just 3 days, compared to 4 days for Delta. The serial interval, which is the time between one person getting sick and the person they infected getting sick, is even shorter: 2 days for BA.1, 3 days for BA.2, and 4 days for Delta. That compressed timeline means Omicron generations stack on top of each other faster, producing steeper surges in shorter windows.
There’s a particularly important consequence of that math. When the serial interval is shorter than the incubation period, people are spreading the virus before they develop symptoms. This makes contact tracing nearly impossible, because secondary cases may already be infectious by the time the original case realizes they’re sick.
Most Spread Comes From a Small Number of People
Coronavirus doesn’t spread evenly from person to person. Roughly 20% of infected individuals generate about 80% of all secondary infections. This “superspreading” pattern means that most people who catch the virus pass it to few or no others, while a small fraction ignite large clusters. The statistical measure that captures this unevenness, called the dispersion parameter, was estimated at about 0.5 during the UK’s first lockdown, indicating highly concentrated transmission. Pre-pandemic contact patterns pushed that number higher (around 1.1), meaning more people contributed to spread when social mixing was normal.
What drives a superspreading event is a combination of high viral load, many close contacts, and time spent in shared indoor spaces. Before the pandemic, people averaged about 12 daily contacts in the UK. During lockdowns, that dropped to roughly 6, and the variation between individuals increased. Some people still had many contacts while most had very few, which is why targeted settings like workplaces, gatherings, and households remained hotspots even when overall mobility dropped.
Seasonal Surge Patterns
In the United States, SARS-CoV-2 has settled into a consistent two-peak pattern since 2020. Cases rise during the late summer (July through September) and again in winter (December through February). These six months account for about 65% of all detected infections, even though they make up only half the year. The pattern holds across most regions, with the only notable exception being the first pandemic winter in 2020-21, when the national peak arrived in late November rather than December.
This biannual rhythm likely reflects a combination of factors: people gathering indoors during extreme heat and cold, school calendars restarting, holiday travel in winter, and the virus itself evolving rapidly enough to produce new immune-evading variants on a roughly semiannual cycle. The late-summer surge surprises many people who associate respiratory viruses exclusively with cold weather, but indoor crowding during air-conditioned summer months appears to play a similar role.
How Immunity Slows Transmission
Population-level immunity has fundamentally changed how fast the virus can spread in practice. The R0 values cited for Omicron describe spread in a fully susceptible population, but virtually no one is fully susceptible anymore. Most people carry some combination of immunity from prior infections, vaccination, or both.
Hybrid immunity, the combination of vaccination plus at least one prior infection, provides the strongest protection. In a large European study of healthcare workers tracked from 2021 through 2024, hybrid immunity reduced the risk of confirmed infection by 63% during the BA.1/BA.2 wave, 64% during BA.4/BA.5, and 47% during the XBB.1.5/BA.2.86 period. Protection weakened somewhat against newer sublineages, but remained significant. Vaccination alone, without prior infection, showed no statistically significant protection against infection by later variants, though it still reduced severe disease.
This layered immunity is the main reason that even though Omicron subvariants are far more inherently transmissible than the original virus, the actual speed of spread in most communities stays well below what the raw R0 numbers would suggest. The effective reproduction number, which accounts for existing immunity, hovers much closer to 1 during most periods, spiking above it during seasonal surges when new variants arrive and immunity from previous waves has waned.
What Wastewater Tells Us That Case Counts Don’t
With most countries scaling back clinical testing, wastewater monitoring has become one of the most reliable ways to track how fast the virus is actually moving through communities. Sewage sampling detects viral genetic material shed by anyone who is infected, whether or not they have symptoms or take a test. It picks up changes in community transmission earlier than hospital admissions or reported cases, sometimes by one to two weeks.
The CDC’s wastewater dashboard covers hundreds of sampling sites across the U.S. and provides a real-time picture of viral activity at the national, regional, and local level. When wastewater viral concentrations rise sharply, it signals that spread is accelerating in that area, even if official case numbers haven’t caught up yet. For anyone trying to gauge current risk in their community, wastewater data is generally more informative than reported case counts.