27th November 2020, Dr Chee L Khoo
Coronaviruses compose a family within the Nidovirales order and replicate by use of a nested set of mRNAs. Four seasonal human coronaviruses (229E, NL63, OC43, and HKU1) have been identified as causing up to a third of community-acquired upper respiratory tract infections. Yet our knowledge about the immune response against this family of viruses is very limited. We know that immunity to seasonal coronaviruses tends to be short in duration lasting from 2-3 months to a few years. We also know that reinfection occurs with Covid-19 and other coronaviruses. Whether such reinfection represents non-durable protective immunity, different strains of the same virus, or both, is unclear (2). What do we know thus far about the immune response following Covid-19 infection? How useful will vaccines be and what about their potential for harm?
What do the presence of antibodies mean?
We know that in patients infected with either severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1) or Middle East respiratory syndrome coronavirus, antibodies against the virus were measurable for 2–3 years, but these markers were absent when patients were re-tested 5–6 years later (3,4). With Covid-19, post-infection immunity is unknown and biological and genetic determinants of disease severity remain unclear. There are also T-cell responses in patients who had Covdi-19 but the significance of those responses is also unclear.
Antibody (humoral) responses to Covid-19 are mediated by antibodies that are directed to viral surface glycoproteins, mainly the spike glycoprotein (so-called spike or S protein) and the nucleocapsid protein. The spike protein contains two sub-units (S1 and S2). The S1 subunit holds a receptor-binding domain (RBD) which is what the virus uses to bind to the human ACE2 receptor on the surface of cells and is the main target for Covid-192 neutralising antibodies.
Antibody titres are good biomarkers for the protective efficacy of antibodies and successful humoral immune responses after Covid-19 exposure. Indeed, most patients with COVID-19 or those who are convalescent have virus-specific IgM, IgA, and IgG responses in the days after infection, suggesting that antibodies mediate protective immunity to Covid-19 (12,13). IgA antibodies were produced in the first week and peaked in concentration at 20–22 days, whereas IgM antibodies reached high titres at 10–12 days and waned 18 days after the onset of symptoms. IgG titres increased during the first 3 weeks and began to decrease by 8 weeks (15).
In patients with mild COVID-19, a rapid decline of RBD-specific IgG titres within 2–4 months has been observed in several studies, suggesting that Covid-19-induced humoral immunity might not be long-lasting in individuals with mild disease.
Antibody titres and disease severity – more doesn’t mean better!
Antibody titres were significantly higher in patients with severe disease than they were in patients without severe disease and were associated with more severe disease (19). On the other hand, patients with COVID-19 and low IgG titres had a higher rate of viral clearance than did patients with COVID-19 and high IgG titres. This suggests that strong antibody responses might be associated with more severe disease, and low antibody responses might be associated with higher rates of viral clearance (19).
However, in a comprehensive study of adaptive immunity to SARS-CoV-2, which also examined the association with disease severity, the concentration of neutralising antibody was not correlated with COVID-19 severity. They suggest that perhaps, cellular (T-cell) responses are important in resolving infection.
Antibody titres – how long do they last?
Some studies have reported that infected individuals become seronegative within 4 months of infection (16, 20, 21) especially in patients who are asymptomatic. However, studies from Iceland reported that 91·1% of individuals who tested positive for Covid-19 remained seropositive 4 months following diagnosis, with no reduction in antibody titres (22).
T-cell response to Covid-19
The proportion of both CD4+ and CD8+ increases during the first 7–10 days of COVID-19 symptoms and begins to return to baseline around day 20. Serious illness has also been linked to a greater reduction in peripheral CD4+ and CD8+ T cell counts compared with non-serious illness, suggesting a link between disease severity and the size of the cellular immune response. The CD4+ T-cell response predominantly consisted of T-helper-1 cells, characterised by high concentrations of IFNγ secretion. CD8+ T-cell responses produced IFNγ and tumour necrosis factor-α, also reflective of a response skewed towards Th1 cells.
Two small studies 37,51 have also suggested that some individuals exposed to SARS-CoV-2 develop specific T-cell memory responses in the absence of specific antibodies, indicating that cellular immunity can be induced by Covid-19 in the absence of antibody responses. The contribution of cellular immunity to protection against COVID-19 is not currently clear. A balanced humoral and Th1-directed cellular immune response might be important for protection from COVID-19 and the avoidance of vaccine-enhanced disease (52).
The vaccine candidates
Various candidate vaccines are being developed and tested, including nucleic acid vaccines, inactivated virus vaccines, live attenuated vaccines, protein or peptide subunit vaccines, and viral-vectored vaccines. Each approach has advantages and disadvantages. The front runner candidates are all administered by the intramuscular route. Therefore, focus is on evaluating immune responses in the blood rather than those in the mucosal surfaces. The role of mucosal immunity should not be discounted.
Naturally, all the candidate vaccines have passed Phase 1 and 2 safety trials and have reported no major adverse side effects. None have completed their Phase 3 efficacy trials but there are all in the pipeline. Oxford Uni, Moderna and Pfizer have issue preliminary announcement but none have had their data peer reviewed.
They have developed a chimpanzee adenovirus-vectored investigational vaccine encoding the spike glycoprotein of Covid-19 (62). Induction of humoral and cellular responses, characterised by anti-spike glycoprotein IgG and neutralising antibodies and IFN. T-cell responses were seen in most recipients after the first dose of vaccine and an additional increase in humoral immune outcomes after the second dose of vaccine. There were no major adverse events althout the phase 3 trial was paused after a vaccine recipient developed symptoms consistent with transverse myelitis. Although the UK trial resumed shortly after the pause, as of Oct 5, 2020, the US trial has not yet resumed. This vaccine requires refrigeration, which could be problematic for use in low-income countries.
Moderna and the National Institutes of Health have jointly developed an mRNA-based vaccine consisting of a sequence-optimised mRNA encoding the spike glycoprotein (64). These humoral immune responses were similar to those observed in convalescent plasma from patients who had recovered from COVID-19. Vaccine recipients also developed cellular responses, mainly biased towards CD4+ Th1 cells. One potential issue for vaccine deployment is that a storage temperature of –20°C is required.
Pfizer and BioNtech
Pfizer and BioNtech have also developed a series of mRNA-based COVID-19 vaccines. Early results from phase 1/2 trials 66 testing two vaccines (BNT162b1 and BNT162b2). BNT162b1 was found to have more local side-effects and BNT162b2 is selected for Phase 3 trials. BNT162b1 and BNT162b2 elicited similar dose-dependent titres of neutralising antibody but BNT162b2 elicited a higher T-cell response. BNT162b2 requires storage at –80°C.
Johnson and Johnson
They have developed a replication-defective Ad26.COV2.S vaccine, which expresses full-length spike glycoprotein (67). Results have shown that a single immunisation induces strong neutralising antibody responses and provides protection against Covid-19 challenge in rhesus macaques aged 6–12 years (68). This candidate vaccine, which requires storage at 2–8°C, is now being tested in a phase 1/2 trial involving 1045 participants (aged 18–55 years and ≥65 years) in the USA and Belgium
The Gamaleya National Research Centre for Epidemiology and Microbiology (Russian Federation) have published the results of two phase 1/2 clinical trials of their COVID-19 vaccine consisting of recombinant adenovirus serotype 26 vector and recombinant adenovirus serotype 5 vector, both carrying the gene for the SARS-CoV-2 spike glycoprotein (69). The vaccine has been tested in 76 healthy individuals aged 18–60 years (38 participants in each study).
Both humoral and cell immune responses were detected in all volunteers and peaked at day 28 after vaccination. The Institute of Biology at the Academy of Military Medical Sciences announced the approval of their adenovirus-vectored vaccine (Sputnik V) in August 2020, before the phase 3 clinical studies had started.
This China-based company have developed a recombinant adenovirus serotype 5-vectored COVID-19 vaccine that expresses the Covid-19 full-length spike glycoprotein from the Wuhan-Hu-1 virus strain (38). Neutralising antibody titres increased by at least four times from baseline in 11 (31%) of 36 participants in the middle dose group at day 14 and in 18 (50%) at day 28, and in 15 (42%) of 36 participants in the high-dose group at day 14 and in 27 (75%) at day 28. They announced the approval of their vaccine in June 2020, before the start of phase 3 testing (71). The phase 3 trial involving 40 000 participants is underway in Pakistan and China.
This vaccine (CoronaVac) is a chemically inactivated, whole-virus preparation administered in a two-dose regimen (at day 0 and day 28) and was granted an emergency use authorisation by Chinese authorities in July, 2020, before the initiation of phase 3 studies. In phase 1/ 2 trials, no serious adverse events were recorded. The vaccine elicited anti-RBD antibodies, as measured by ELISA, and neutralising antibodies 14 days after the second dose of vaccine in 92·4% of individuals receiving the vaccine at 0 and 14 days, and in 97·4% of those receiving the vaccine at 0 and 28 days. a two-dose regimen (at day 0 and day 28) and was granted an emergency use authorisation by Chinese authorities in July, 2020, before the initiation of phase 3 studies. A phase 3 trial has been launched in Brazil and Indonesia, with the trial in Brazil aiming to enrol 9000 health-care personnel.
Sinopharm have developed and are testing two inactivated whole-virus, alum-adjuvanted vaccines. Phase 1/2 trials indicate that the vaccines were safe and neutralising antibodies were similar in concentration to those produced by other COVID-19 vaccines. There were no cellular response generated. In late August, 2020, Sinopharm researchers revealed that they had already begun to administer the vaccine to health-care personnel and groups at high risk of becoming infected. A phase 3 clinical trial began in July, 2020, and plans to enrol 21 000 participants in the United Arab Emirates, Bahrain, Peru, Morocco, Argentina, and Jordan.
Vaccine enhancement of disease – is this a concern?
While inducing neutralising antibodies is useful in developing protection against Covid-19, antibodies might have the potential to facilitate viral entry into cells through interactions with Fc receptors or complement. Even in the absence of active viral replication in immune cells, this process might lead to the activation of macrophages, monocytes, and B cells, and IL-6, TNFα, and IL-10 production.77 Cases of antibody-dependent enhancement induced by vaccines have been reported after the use of formalin-inactivated vaccines against respiratory syncytial virus and measles, and after the use of a vaccine against dengue virus.78–80 Concerns have therefore been raised regarding the potential for antibody-dependent enhancement in individuals who are infected with SARS-CoV-2 after vaccination with a COVID-19 candidate vaccine.38
The potential risk of antibody-dependent enhancement mediated by Fc receptors could be increased with mutations in the SARS-CoV-2 spike glycoprotein, which could weaken the primary host antibody response. Monocyte, macrophage, and B-cell infection might occur in numerous tissues as a result of subsequently unstable virus-antibody complexes, leading to extensive apoptosis of immune cells and the production of inflammatory cytokines.52
Antibody-dependent enhancement poses a theoretical obstacle to vaccine development and is being carefully evaluated.52,87 The extent to which pre-existing antibodies to SARS-CoV-2 (and potentially to SARS-CoV-1) might contribute to antibody-dependent enhancement and disease severity remains in question;81 however, no evidence of antibody-dependent enhancement has been found in animal models or in humans in phase 3 clinical trials.
- So far we only have announcements from the various companies but none of their data have been released for review. We don’t know what “90% protection” means. These are not statistical terms. It seems from the limited announcements that they are claiming that their vaccines reduce the number of patients with symptoms. They are not claiming that their vaccine prevent infection. They are just claiming reduction of the number of patients with symptoms. Does that mean that vaccination reduce the number of people with severe disease but they may become asymptomatic carriers?
- So far, we also don’t know the endurance of the protection? If they are raising neutralising antibodies with the vaccines, will they disappear by 3-4 months like in patients with the real Covid-19 infection?
- As patients with severe disease have severe disease because of enhanced inflammatory process, will vaccination cause severe disease in patients who already have previous mild covid-19 infection?
- Most previous vaccines development take 6 months of large-scale use before being approved by the regulatory authorities. There are usually further 5-6 further supplementary trials beyond the first phase 3 trials as other investigators corroborate the findings. Is there a danger that this approval process is now shortened to 2-3 months and based on data from only one trial ?
Gregory A Poland, Inna G Ovsyannikova, Richard B Kennedy. SARS-CoV-2 immunity: review and applications to phase 3 vaccine candidates. Lancet Published Online October 13, 2020 https://doi.org/10.1016/ S0140-6736(20)32137-1