Paradoxical vaccines

gene stops hereA new breed of vaccines is on the horizon: they replicate in one type of cell, allowing for their production, but will not replicate in humans. Two different examples have recently been described for influenza and chikungunya viruses.

The influenza virus vaccine is produced by introducing multiple amber (UAG) translation stop codons in multiple viral genes. Cloned DNA copies of the mutated viral RNAs are not infectious in normal cells. However, when introduced into specially engineered ‘suppressor’ cells that can insert an amino acid at each amber stop codon, infectious viruses can be produced. These viruses will only replicate in the suppressor cells, not in normal cells, because the stop codons lead to the production of short proteins which do not function properly.

When inoculated into mice, the stop-codon containing influenza viruses infect cells, and although they do not replicate, a strong and protective immune response is induced. Because the viral genomes contain multiple mutations, the viruses are far less likely than traditional infectious, attenuated vaccines to sustain mutations that allow them to replicate in normal cells. It’s a clever approach to designing an infectious, but replication-incompetent vaccine (for more discussion, listen to TWiV #420).

Another approach is exemplified by an experimental vaccine against chikungunya virus. The authors utilize Eilat virus, a virus that only replicates in insects. The genes encoding the structural proteins of Eilat virus were replaced with those of chikungunya virus. The recombinant virus replicates in insect cells, but not in mammalian cells. The virus enters the latter cells, and some viral proteins are produced, but genome replication does not take place.

When the Eilat-Chikungunya recombinant virus in inoculated into mice, there is no genome replication, but a strong and protective immune response is induced. The block to replication – viral RNA synthesis does not occur – is not overcome by multiple passages in mice. Like the stop-codon containing influenza viruses, the Eilat recombinant virus is a replication-incompetent vaccine.

These are two different approaches to making viruses that replicate in specific cells in culture – the suppressor cells for influenza virus, and insect cells for Eilat virus. When inoculated into non-suppressor cells (influenza virus) or non-insect cells (Eilat virus), a strong immune response is initiated. Neither virus should replicate in humans, but clinical trials have to be done to determine if they are immunogenic and protective.

The advantage of these vaccine candidates compared with inactivated vaccines is that they enter cells and produce some viral proteins, likely resulting in a stronger immune response. Compared with infectious, attenuated vaccines, they are far less likely to revert to virulence, and are easier to isolate.

These two potential vaccine technologies have been demonstrated with influenza and chikungunya viruses, but they can be used for other virus. The stop-codon approach is more universally applicable, because the mutations can be introduced into the genome of any virus. The Eilat virus approach can only be used with viruses whose structural proteins are compatible with the vector – probably only togaviruses and flaviviruses. A similar approach might be used with insect-specific viruses in other virus families.

Why do I call these vaccines ‘paradoxical’? Because they are infectious and non-infectious, depending on the host cell that is used.

Note: The illustration is from a t-shirt, and the single letter code of the protein spells out a message. However the title, ‘the gene stops here’, is wrong. It should be ‘the protein stops here. The 3’-untranslated region, which continues beyond the stop codon, is considered part of the gene.

TWiV 420: Orthogonal vectors

The TWiV gurus describe how to use an orthogonal translation system to produce infectious but replication-incompetent influenza vaccines.

You can find TWiV #420 at microbe.tv/twiv, or listen below.

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Effectiveness of this season’s influenza vaccine

influenza virusThe Centers for Disease Control and Prevention has released early estimates of the effectiveness of this season’s influenza vaccine in preventing influenza infection.

Influenza vaccine effectiveness is assessed each year by the U.S. Influenza Vaccine Effectiveness Network. Patients with acute respiratory infection (ARI) are enrolled in the study; respiratory samples are taken and the presence of influenza virus is determined by polymerase chain reaction.

Data from 1,155 children and adults with ARI during December 3, 2012–January 2, 2013 were collected at five study sites and used to determine that the estimated vaccine effectiveness is 62% (95% confidence intervals = 51%–71%). This number represents the overall effectiveness of seasonal influenza vaccine for preventing laboratory-confirmed influenza virus infection associated with acute respiratory infection.

Of 1,155 children and adults with ARI, 416 (36%) were positive for influenza A or B virus. Of these, 236 (57%) were influenza A virus (all H3N2) and 180 (43%) were influenza B viruses. The immunization rate for influenza cases was 32% and 56% for individuals who did not have influenza virus. The ARI of these patients was likely caused by another agent.

It is known that this season’s influenza vaccine is a good match for the circulating viruses. Why then is vaccine effectiveness only 62%? Although the study does not distinguish between patients who received the inactivated (injected) or the infectious (intranasal) vaccine, most vaccine distributed in the US is the former. Because the inactivated vaccine is treated with formaldehyde and detergents, the viral proteins are not in their native state and do not produce the correct antibodies for optimal protection. Because the intranasal vaccine contains infectious virus, the viral proteins are unaltered and induce antibodies that are more protective. In children less than 7 years old, the intranasal vaccine provides a higher level of protection than the inactivated vaccine. It’s unfortunate that the influenza vaccine needs of the US cannot be met by current supplies of FluMist.

Based on the results of this and similar studies, some argue that it is not worth being immunized against influenza. This reasoning is faulty. Even if you are immunized and subsequently contract influenza, you are likely to be less sick than if you had not received the vaccine. According to CDC:

Influenza vaccination, even with moderate effectiveness, has been shown to reduce illness, antibiotic use, doctor visits, time lost from work, hospitalizations, and deaths.