The COVID-19 pandemic has sent scientists to the drawing board with a vengeance, to construct an effective and affordable vaccine that can be rapidly produced at mass scale. Over a hundred vaccines are being developed, according to the World Health Organization (WHO). Now a new study published in the journal Nature Communications describes a vaccine candidate based on an adenoviral vector that protects against infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
The researchers selected the human adenovirus type 5 on which to base their vaccine because this replication-defective vector had already been used in their Ebola vaccine, with an excellent safety and immunogenicity profile in a phase 1 trial. The current study aimed to produce a vaccine that could offer mucosal vaccination and enhanced protection against this respiratory virus.
Comparing Vaccination Routes
The study thus compares the effectiveness of vaccination by the mucosal vs. intramuscular route, concerning antibody induction, T cell responses, and the efficacy of the vectored vaccine in an animal model following a challenge by SARS-CoV-2, when the vaccine is used either intranasally or intramuscularly.
The antigen was constructed using prior research, based on the full-length spike protein of the Wuhan-Hu-1 strain. The spike gene was optimized so that it would induce elevated antigen expression in mammalian cells. The researchers decided to use the AD5 replication-defective virus lacking E1/E3 expressing the full-length Spike protein led by a signal peptide comprising the tissue plasminogen activator (tPA).
They used wildtype BALB/c mice and ferrets to test the protective efficacy of the vaccine. The mouse is an excellent animal model for this virus since it supports infection in both upper and lower respiratory tracts. The mice were 6-8-week-old females at the time of the study. They were immunized with one dose of 5 × 109 virus particles (VP) (high dose), 5 × 108 VP (middle dose), or 5 × 107 VP (low dose) of Ad5-nCoV. Controls were given 5 × 109 VP of the control vaccine (Ad5 vector). All doses were given by the intramuscular (IM) or intranasal (IN) route
Strong Adaptive Immune Response
The study shows that this vectored virus Ad5-nCoV induces a robust immune response, both in terms of antibody production and T cell responses. Both groups show specific anti-S IgG, IgG1 and IgG2 antibody, neutralizing antibody and IgA, as well as a strong cellular immune response. The IgG levels were highest on day 28, when IM immunization was administered. IN immunization resulted in a steady elevation of IgG between weeks 4 and 8.
With higher IN doses, the IgG titers were higher relative to the intramuscular group at week 6 and week 8. However, at other dose levels, there was no difference with the route of administration at either point.
Irrespective of route of inoculation, potent and specific IgG1 and IgG2a antibodies were produced, but more with the IM route. Neutralizing antibodies (NAbs) peaked at week 6 and 8 with the IN and IM routes, respectively. High IN doses produced higher NAbs than with the IM route between week 4 and week 8, which was not seen with low or middle doses. Similar results were seen with the pseudovirus.
IgG antibodies targeting the Spike protein of the virus were found in both groups by week 2 and week 10, while specific IgA was found only in the IN group. High doses by either route resulted in the detection of NAbs in tracheal and lung lavage fluid.
With middle doses by either route, CD8 T cells in the spleen and CD4 T cells showed marked production of IFNγ, TNFα, and IL-2 responses. The cellular immune response was found to be positively correlated with the dose used in the IM group at week 10, but not with the IN group.
Protection Against SARS-CoV-2
The researchers found that IN inoculation of 70% of the vaccinated mice in each group with the mouse-adapted SARS-CoV-2 at 103.6 plaque-forming units (PFU) per mouse at week 10 after immunization prevented infection of the nasal or lung epithelium in all IN vaccinated mice, as shown by autopsy results at 3 and 5 days from inoculation. All the control animals were infected, with lung and nasal epithelium showing a mean viral load of 5.6 × 105 PFU/g and 1.2 × 104 PFU/g, respectively.
However, the IM vaccinated mice did show the presence of the virus in the nasal turbinates but not the lungs, with the viral load decreasing in the low and middle dose groups.
The researchers say, “A single low dose of Ad5-nCoV can completely protect both the upper respiratory tract and lungs of mice from infection.’ However, the upper respiratory tract is more difficult to protect than the lungs, as seen by the incomplete protection seen with the IM route of vaccination.
Protection against Wildtype Virus in Ferrets
The ferret is highly susceptible to infection with the virus in the upper respiratory tract. Thus the researchers tested whether it could be protected in the upper respiratory tract by IN + Oral vaccination in one group and IM vaccination in the other group of ferrets.
They found that when challenged at week 4 from immunization, all vaccinated ferrets had anti-S IgG and NAbs but not the controls. Both groups showed comparable responses. Cell immunity was seen in most ferrets in the IM and half the ferrets in the mucosal vaccination groups, respectively.
No virus was found in nasal wash fluid, by either PCR or PFU tests, in any of the mucosal vaccinated animals, between day 2 and day 8 from infection, while all controls showed infection. The IM group did have a virus in the nose washes in half the animals on day 2, one in three on day 4, but none thereafter. The viral load in the IM group also was markedly reduced by day 4, 6, and 8 relative to controls.
The study shows for the first time that one mucosal dose of vaccine produces complete protection of the upper and lower respiratory tract against SARS-CoV-2. One IM dose can protect the lower respiratory tract in mice and reduce viral load in both mice and ferrets exposed to infection. The titers may correlate with protective ability, as shown in rhesus monkeys.
The researchers call for human trials to validate the protection provided by IM or mucosal vaccination in humans exposed at the natural dose (compared to the very high doses of virus used in this trial to challenge vaccinated animals). The importance of mucosal immunity, mediated by anti-S antibodies or vaccine-induced tissue-resident T cells, is the first-line defense it provides to the body against a pathogen, apart from systemic immune reactions. Thus, they say, “The mucosal vaccine supply an important advantage that protects against virus replication in the upper respiratory tracts, interrupting person to person transmission.”
Prior research has shown that nasal-spray live attenuated flu vaccine was safe for children and adolescents with asthma, though respiratory viruses are well known to cause asthma exacerbations. This shows the need to monitor the safety profile of this vaccine given by the mucosal route in future trials, as well as to test different vaccination routes to control the current pandemic.