key: cord-0764510-ho11jjcl
authors: Cohen, Jeffrey I; Burbelo, Peter D
title: Reinfection with SARS-CoV-2: Implications for Vaccines
date: 2020-12-18
journal: Clin Infect Dis
DOI: 10.1093/cid/ciaa1866
sha: cdc6f4a57cd20cc7880c6fe0da9b0f0eef0da2b4
doc_id: 764510
cord_uid: ho11jjcl

Infection with SARS-CoV-2 has become pandemic and the duration of protective immunity to the virus is unknown. Cases of persons reinfected with the virus are being reported with increasing frequency. At present it is unclear how common reinfection with SARS-CoV-2 is and how long serum antibodies and virus-specific T cells persist after infection. For many other respiratory virus infections, including influenza and the seasonal coronaviruses that cause colds, serum antibodies persist for only months to a few years and reinfections are very common. Here we review what is known about the duration of immunity and reinfection with coronaviruses, including SARS-CoV-2, and well as the duration of immunity to other viruses and virus vaccines. These findings have implications for the need of continued protective measures and for vaccines for persons previously infected with SARS-CoV-2.

Most of what is known about immune responses to virus infections pertains to the duration of the antibody response. Viruses that result in a systemic infection with viremia usually induce long lived antibody responses that last for decade or more [2, 3] (Table 1 ). In contrast, viruses that infect mucosal surfaces and do not have a viremic phase typically result in antibody responses that are detected for months or a few years. These latter viruses include influenza virus, respiratory syncytial virus, and seasonal coronaviruses. While SARS-CoV-2 RNA has been reported in the blood of infected persons, infectious virus has not been reported in the blood [4] and with the large number of asymptomatic infections, one would expect reports of virus transmission from blood transfusion if viremia was common.

A c c e p t e d M a n u s c r i p t 4 SARS-CoV-2 is a member of the beta-coronaviruses and it is genetically most similar to SARS-CoV-1 (SARS) and MERS. Two studies of neutralizing antibody to SARS-CoV-1 found that these antibodies persist for at least two years, although it is not known if they are protective from infection [5, 6] . Two other studies found that antibody responses were limited in persons with MERS and mild disease compared to those with severe disease [7, 8] ; a third study reported persistent antibody responses at 2 years regardless of severity of symptoms [9] . Both of these coronaviruses have a high mortality rate with severe disease.

Other human beta-coronaviruses, OC43 and HKU1, result in colds and very rarely cause severe disease except in highly immunocompromised persons. Antibody responses to these seasonal coronaviruses often fall sharply within a year after infection and then rise quickly after reinfection [10] .

Most antibodies to SARS-CoV-2 are measured to the nucleocapsid protein, the spike protein (or the receptor binding domain within the spike protein) or as antibodies that neutralize infection with the virus or with pseudotyped viruses. The latter are other viruses, such as lentiviruses or vesicular stomatitis virus, that are engineered to express the spike protein on the surface and are safer to work with than SARS-CoV-2. The largest study to date to determine the duration of antibody responses to SARS-CoV-2 used ELISA assays and showed that antibody levels to the receptor binding domain of the virus rise during the first month after infection and then are relatively stable for the next 2.5 months [11] . Another study showed that antibody titers to the spike protein declined by <2-fold between days 30 to 148 after infection [12] . Neutralizing antibody titers to SARS-CoV-2 are likely to be important for protection from infection. Studies of neutralizing antibody responses have often shown a decline in titers over time. In one study, neutralizing antibody levels declined 4-fold from one to four months after the onset of symptoms [13] . Another study showed that the median neutralization titers decreased by 45% over 4 weeks and that patients with the A c c e p t e d M a n u s c r i p t 5 highest neutralizing titers had the largest drop in antibody titers [14] . A third study showed that neutralizing titers reached a peak at 23 days after onset of symptoms and then declined;

persons with more severe disease had higher levels of peak neutralizing titers and still had detectable levels of these antibodies 2 to 3 months after onset of symptoms, while those who were asymptomatic or had mild symptoms had lower levels of peak antibody titers and some fell below the level of detection at 2 months after infection [15] . Since SARS-CoV-2 has been in the human population for less than a year, the long-term duration of antibody responses is unknown.

T cells also have an important role in maintaining long term immunity to viruses. A recent study showed that both CD4 and CD8 SARS-CoV-2-specific T cells are maintained for >6 months after the initial infection [16] . T cells that recognized spike, nucleocapsid, and membrane proteins of the virus were more prevalent than T cells that responded to SARS-CoV-2 accessory proteins. T cells at mucosal sites, particularly tissue resident memory T cells, are especially important to maintain long term immunity for virus infections that enter through mucosal surfaces [17] . In adoptive transfer experiments in which IgG from SARS-CoV-2 convalescent macaques was given to naive animals before virus challenge, antibody was protective from virus challenge, but CD8+ T cells were important when antibody responses were not fully protective [18] . Thus, T cells may also be important for protection from SARS-CoV-2 infection.

Antibodies are a correlate of protection for virtually all vaccines that protect against acute virus infections [19] . Challenge studies with human seasonal coronaviruses, particularly HuCoV 229E, have been instructive in correlating antibody responses to protection from infection, disease, and virus shedding. Persons with neutralizing antibody titers to HuCoV A c c e p t e d M a n u s c r i p t 6 229E had less infection and were more often asymptomatic after challenge [20] . After challenge with HuCoV 229E, neutralizing antibody peaked at 3 weeks and fell considerably at 12 weeks [21] . One year later the volunteers were rechallenged; 6 of 9 volunteers became reinfected and all were asymptomatic. The duration of shedding was one-half to one-third as long as after the initial virus challenge. Seven of 8 persons with neutralizing titers of < 1:5

to HuCoV 229E excreted virus after challenge compared to only 1 of 4 with preexposure titers of >40 [22] . Fewer individuals with >10 3.5 ELISA antibody titers experienced significant colds upon viral challenge with HuCoV 229E than those with lower antibody titers [23] .

Two studies provide evidence that neutralizing antibodies to SARS-CoV-2 may protect against infection [24] . 120 persons on a fishing boat were tested for SARS-CoV-2 antibody and viral RNA with nasopharyngeal swabs before departure and after return (mean 32 days). Eighteen days after departure the ship returned to shore after a person became sick 

At present, with the exception of the reports cited above [24, 25] cells specific for the spike and nucleocapsid proteins were detected 6 months after the onset of symptoms [16, 26] .

The duration of antibody responses to natural infection with viruses noted above, generally mirror the duration of immune protection by vaccines against the corresponding virus. Live attenuated vaccines to viruses that have a viremic stage such as measles, mumps, rubella, hepatitis A, and yellow fever generally provide lifelong protection from disease after vaccination in infancy and/or childhood ( Table 1) 

Reinfection with viruses that causes systemic infections, such as measles, mumps, rubella, hepatitis A virus, yellow fever, and polio (with the same serotype) is very uncommon. In contrast, reinfection with viruses that cause mucosal infections without viremia such as respiratory syncytial virus, influenza, and seasonal coronavirus is common. Repeated episodes of respiratory syncytial virus are common in young children after natural infection [27] and after challenge with the same strain group from previous infection [28] . Similarly, rapid reinfection with influenza after an epidemic [29] as well as reinfection with the identical lot of influenza used in consecutive challenge studies [30] has been reported.

Reinfection with seasonal coronaviruses has been reported based on repeated rises in antibody titers defined as a > 1.4-fold increase [10] . Using this criterion, the mean time to reinfection with the four seasonal coronaviruses was 30 months, ranging from 30-55 months depending on the virus. Using RT-PCR from nasal swabs, 14% (12 of 86) of persons had multiple reinfections with the same seasonal coronavirus [31] . There was no association 

The observation of well documented case reports of reinfection with SARS-CoV-2 reported less than a year since the virus entered the human population has several implications ( Table   3 ). While they represent a tiny fraction of the very large number of cases of COVID-19, these cases may nonetheless represent a small fraction of the number of persons who have actually been reinfected. Documentation of these cases required having sufficient specimen preserved from the first case, and sufficient laboratory support so that strain differences could be verified; all occurred within < 150 days after the initial infection. There is likely a bias for reporting more symptomatic cases of reinfection and additional time will be needed to understand the real frequency of reinfection. In the absence of a potent vaccine or antiviral medication, the finding of patients reinfected with SARS-CoV-2, which in some cases can be as severe or even more severe than the primary infection, implies that precautions including masks and distancing are still important after recovery from COVID-19. In addition, previously infected persons may need vaccination. Herd immunity from infection is unlikely to be sufficient to eliminate the virus if reinfection is common, as it is with seasonal coronaviruses.

Some vaccines, such as the human papillomavirus vaccine provide higher levels of antibodies than with natural infection and persist for years. In contrast, other vaccines such as influenza provide temporary immunity and need to be given yearly. If vaccines to SARS-CoV-2 have efficacy similar to current influenza vaccines, immunization with SARS-CoV-2 may need to be given annually and if so, might be given as a component of annual influenza vaccines.

While most vaccines under development to SARS-CoV-2 are given intramuscularly and show high levels of protection, it is uncertain how long protection will last, whether these A c c e p t e d M a n u s c r i p t 12 vaccines will prevent infection in addition to disease, and whether they will reduce shedding if persons become reinfected. Vaccines that are given by the mucosal route may have an advantage of inducing higher levels of local immunity and may result in tissue resident T and B cells. For example, the live attenuated oral poliovirus vaccine induces high level of neutralizing antibody in the intestine to prevent infection and shedding of wild-type virus, while the inactivated poliovirus vaccine given intramuscular protects against disease, but not infection or shedding [36] . Studies of SARS-CoV-1, SARS-CoV-2, and MERS vaccines in animals indicate that intranasal vaccination was more effective in many studies compared with intramuscular vaccines [37, 38] . Thus, for SARS-CoV-2 a vaccine delivered intranasally, the natural route of infection, might induce better mucosal immunity with local memory B cells and tissue resident memory T cells, and reduce infection and shedding more effectively than a vaccine given intramuscularly. M a n u s c r i p t 20 A c c e p t e d M a n u s c r i p t 21 

Gemelli Against COVID-19 Post-Acute Care Study Group. Persistent Symptoms in Patients After Acute COVID-19

Long-term humoral immunity against viruses: revisiting the issue of plasma cell longevity

Duration of humoral immunity to common viral and vaccine antigens

Risk of transmission of severe acute respiratory syndrome coronavirus 2 by transfusion: A literature review Transfusion

Longitudinal profile of antibodies against SARScoronavirus in SARS patients and their clinical significance

Lack of cross-neutralization by SARS patient sera towards SARS-CoV-2

Antibody Response and Disease Severity in Healthcare Worker MERS Survivors

MERS-CoV Antibody Responses 1 Year after Symptom Onset, South Korea

Recovery from the Middle East respiratory syndrome is associated with antibody and T-cell responses

Seasonal coronavirus protective immunity is short-lasting

Humoral Immune Response to SARS-CoV-2 in Iceland

Robust neutralizing antibodies to SARS-CoV-2 infection persist for months

Longitudinal analysis of clinical serology assay performance and neutralising antibody levels in COVID19 convalescents

Longitudinal observation and decline of neutralizing antibody responses in the three months following SARS-CoV-2 infection in humans

Immunological memory to SARS-CoV-2 assessed for greater than six months after infection

Exploiting Mucosal Immunity for Antiviral Vaccines

Correlates of protection against SARS-CoV-2 in rhesus macaques

Correlates of protection induced by vaccination

Effect of specific humoral immunity and some non-specific factors on resistance of volunteers to respiratory coronavirus infection

The time course of the immune response to experimental coronavirus infection of man

Effects of a "new" human respiratory virus in volunteers

The effect of intranasal nedocromil sodium on viral upper respiratory tract infections in human volunteers

Neutralizing Antibodies Correlate with Protection from SARS-CoV-2 in Humans during a Fishery Vessel Outbreak with a High Attack Rate

Antibodies to SARS-CoV-2 are associated with protection against reinfection

Rapid and lasting generation of B-cell memory to SARS-CoV-2 spike and nucleocapsid proteins in COVID-19 disease and convalescence

Respiratory-syncytialvirus infections, reinfections and immunity. A prospective, longitudinal study in young children

Immunity to and frequency of reinfection with respiratory syncytial virus

An epidemic of influenza on Tristan da Cunha

Influenza A Reinfection in Sequential Human Challenge: Implications for Protective Immunity and "Universal" Vaccine Development

Direct Observation of Repeated Infections With Endemic Coronaviruses

Setting the criteria for SARS-CoV-2 reinfection -six possible cases

Centers for Disease Control and Prevention. Investigative Criteria for Suspected Cases of SARS-CoV-2 Reinfection (ICR) Available at

COVID-19 re-infection by a phylogenetically distinct SARS-coronavirus-2 strain confirmed by whole genome sequencing

Development of SARS-CoV-2 vaccines: should we focus on mucosal immunity?

A single dose intranasal ChAd vaccine protects upper and lower respiratory tracts against SARS-CoV-2

A c c e p t e d M a n u s c r i p t 14