As the COVID-19 pandemic has continued, getting infected more than once has become a documented reality. Initial expectations of long-lasting immunity after an infection have been tempered by the virus’s capacity to change. Understanding reinfection involves looking at how the virus evolves and how our immune systems respond over time. This reality has shifted the public health conversation towards managing ongoing exposure to this persistent virus.
Defining and Measuring Reinfection
Public health organizations have established specific criteria to distinguish a new infection from a lingering one. The U.S. Centers for Disease Control and Prevention (CDC) considers a new positive test 90 days or more after an initial infection to be a reinfection. This interval helps account for prolonged viral shedding, where remnants of the virus can be detected by sensitive PCR tests weeks after an infection has resolved.
Confirming reinfection with the highest confidence involves genomic sequencing. This laboratory analysis can determine if the virus from the second illness belongs to a different genetic lineage than the first. By comparing the genetic makeup of the two samples, scientists can confirm the second case is a distinct event and not a resurgence of the original infection.
However, tracking the true rate of reinfection is challenging. The widespread availability of at-home rapid tests means that a significant number of positive results are never officially reported to health authorities. Furthermore, many mild or asymptomatic reinfections may go unnoticed, leading to an undercount of cases. These measurement issues make it difficult to establish a precise reinfection rate.
Key Factors That Influence Reinfection
The primary driver behind reinfections is the evolution of the SARS-CoV-2 virus. New variants, particularly those in the Omicron family, possess mutations in their spike protein. These genetic changes alter the protein’s shape, making it less recognizable to antibodies from a previous infection or vaccination. This immune evasion is a key reason why someone can be susceptible to infection again.
The source of a person’s immunity also plays a part in their defense. Natural immunity from a prior infection, immunity from vaccination, and hybrid immunity—the protection from both vaccination and a past infection—all provide a shield. Hybrid immunity is often considered more robust, offering a stronger defense against a wider range of variants. However, no form of immunity offers complete protection against subsequent infection.
Protection from past infections and vaccines naturally decreases over time. This process, known as waning immunity, means that protective antibody levels in the body decline over the months following an infection or a shot. This reduction in circulating antibodies makes a person more susceptible to a new infection, especially when a new, immune-evasive variant becomes dominant.
Severity and Health Outcomes of Reinfections
For many people, a reinfection with COVID-19 tends to be less severe than their first encounter with the virus. This is because the immune system, having been primed by the initial infection or vaccination, can mount a faster response. This existing memory helps protect against severe outcomes like hospitalization, although factors such as the specific variant, time since the last exposure, and a person’s underlying health conditions can all influence the experience.
Despite the tendency for milder illness, reinfection is not without risks. A study funded by the National Institutes of Health (NIH) found a link between the severity of the first infection and the second. Individuals who experienced severe illness initially were more likely to have a severe reinfection. This suggests that individual responses to the virus can be consistent across multiple encounters.
The risk of developing post-COVID conditions, often called Long COVID, persists with reinfection, though the likelihood may be lower compared to a primary infection. Studies have shown that while the risk of Long COVID is reduced with a second infection, it is not eliminated. Symptoms like fatigue, cognitive issues, and shortness of breath can still develop, meaning that each new infection carries a potential for long-term health consequences.
Evolving Reinfection Rates With New Variants
The rate of COVID-19 reinfection has not been static; it has changed significantly with the emergence of new viral variants. During the periods when the original, Alpha, and Delta variants were dominant, reinfections were relatively uncommon. The protection afforded by a previous infection was quite strong against these earlier forms of the virus, and the reinfection rate was estimated to be low.
This situation changed with the arrival of the Omicron variant in late 2021. Because Omicron and its sublineages can more effectively evade the immune system, the risk of reinfection increased substantially. Data from the UK’s Office for National Statistics indicated that the risk of reinfection was 16 times higher during the initial Omicron wave compared to the period when Delta was dominant.
This sharp increase highlights how viral evolution directly impacts population-level immunity. While the pooled global reinfection rate before Omicron was less than 1%, it jumped to over 4% with Omicron’s emergence. Studies comparing the variants directly found that the risk of reinfection during the Omicron wave was significantly higher than during the Delta wave.