- Rapid decline of neutralizing antibodies following COVID-19 infection in some people have led to concerns that immunity to the SARS-CoV-2 virus Is temporary.
- However, several recent studies have shown a presence of virus-specific T-cells that remain, including among patients with mild or asymptomatic infections.
- Long-lasting immunity after recovery from COVID-19 infection may be significantly higher than antibody testing alone would indicate and contribute to population based herd immunity.
Research into the immune response to SARS-CoV-2 infection has primarily focused on analyzing the blood of patients, who have recovered from COVID-19, to measure whether they have developed neutralizing antibodies to the virus. A key finding early in the pandemic was that any antibodies produced tend to be very short-lived.1
Several recent studies, however, have shown “encouraging signs of strong, lasting immunity,” including in patients who have had mild cases of COVID-19.2 One of the studies, which has not yet been peer reviewed, reported finding that, “immunity is durable for at least several months after SARS-CoV-2 infection.”3
A study currently under review by the journal Nature, found that, “recovered individuals developed SARS-CoV-2-specific IgG (Immunoglobulin G ) antibody and neutralizing plasma, as well as virus-specific memory B- and T-cells that not only persisted, but in some cases increased numerically over three months following symptom onset.”4 This is the information researchers have been looking for to determine whether those who recover from mild or asymptomatic COVID-19 develop a strong and durable immunity that could lead to reduced transmission and ultimately to true population based herd immunity.5
Antibodies are Not the Only Important Players in Developing Immunity
As explained by viral immunologist Zania Stamataki, PhD, there are two major types of “memory” immune responses that allow the body to recognize and quickly mount a protective response to a microbial infection.6 Among many other tools available in the complex human immune system, one type of response is powered by B-cells, which produce antibodies. Another type is the T-cell response, which may involved either a “killer T-cell” that causes the destruction of infected cells or a “helper T-cell” that triggers B-cells to produce new antibodies.7
T-cell activation is reportedly common in patients with COVID-19 and has been seen in early results of vaccine trials. T-cell activation may be able to prevent a second infection even in the absence of antibodies. If SARS-CoV-2 follows the pattern of other coronaviruses, memory T-cells may last longer than antibodies.8
A study conducted by the Karolinska Institute in Sweden found that for every one person who tested positive for antibodies to SARS-CoV-2, two were found to have T-cells specific for the virus. The study also found that patients with severe disease were likely to develop higher levels of antibodies and T-cells, while those with mild or asymptomatic disease generally had lower levels of antibodies but still showed a strong T-cell response. Routine antibody testing would fail to identify patients who do not have antibodies but do have the T-cells that could provide protection from COVID-19 infection and transmission.9
According to Hans-Gustaf Ljunggren, MD, PhD, senior co-author of the Swedish study:
Our results indicate that public immunity to COVID-19 is probably significantly higher than antibody tests have suggested… If this is the case, it is of course very good news from a public health perspective.10
T-Cell Cross-Reactivity May Be Linked to Infection With the Common Cold
Another preprint study out of the Charité University Hospital in Berlin, Germany found that 15 of 18 hospitalized patients had helper T-cells specific to the spike protein on the virus. Questioning whether those who have not been infected with SARS-CoV-2 might still produce the cells to fight infection, Andreas Thiel, PhD and fellow researchers found that 34 percent of 68 uninfected people had helper T-cells able to recognize SARS-CoV-2. This cross-reactivity was also identified in about half of stored blood samples from 2015 to 2018—before the world had heard of COVID-19.
The researchers suggested that these cells were probably generated in response to a previous infection with one of the four coronaviruses that cause the common cold, explaining that proteins in the other coronaviruses resemble those of SARS-CoV-2.
Viral immunologist Steven Varga, PhD of the University of Iowa proposes that, “one reason that a large chunk of the population may be able to deal with the virus is that we may have some small residual immunity from our exposure to common cold viruses.”11
1 Smith C. Coronavirus Immunity Might Be Even Higher Than Tests Show. BGR July 2, 2020.
2 Wu KJ. Scientists See Signs of Lasting Immunity to Covid-19, Even After Mild Infections. The New York Times Aug. 16, 2020.
3 Ripperger TJ, et al. Detection, Prevalence, And Duration Of Humoral Responses To SARS-Cov-2 Under Conditions Of Limited Population Exposure. MedRxiv Aug. 16, 2020.
4 Rodda LB, et al. Functional SARS-Cov-2-Specific Immune Memory Persists After Mild COVID-19. MedRxiv Aug. 15, 2020.
6 Stamataki Z. I’m A Viral Immunologist. Here’s What Antibody Tests For Covid-19 Tell Us. The Guardian June 25, 2020.
7 See Footnote 2.
8 See Footnote 6.
9 See Footnote 1.
10 Sekine T, et al. Robust T Cell Immunity In Convalescent Individuals With Asymptomatic Or Mild COVID-19. Cell (currently in press).
11 Leslie M. T Cells Found In COVID-19 Patients ‘Bode Well’ For Long-Term Immunity. Science May 14, 2020.