Influenza infections during pregnancy: association with congenital malformations and with subsequent neoplasms in children, and potential hazards of live virus vaccines

Is routine influenza immunization warranted in early pregnancy?

Routine influenza immunization is recommended for select groups because of their higher risk of serious influenza outcomes. Based on that benefit-risk framework, we assessed whether routine administration of trivalent inactivated influenza vaccine (TIV) is warranted in pregnancy, beginning in 1st trimester. Higher maternal mortality due to influenza was extensively described during the 1918 and 1957 pandemics, but epidemiologic evidence thereafter is limited to case reports and a single ecologic analysis during a single season. Significantly elevated rates of hospitalization have been reported with seasonal influenza beginning in 1st trimester among women with select comorbidities and during the 2nd half of normal pregnancy. TIV protection against serious outcomes in pregnant women has not yet been shown. Although harm has also not been shown, sample size to date is insufficient to assert TIV safety in 1st trimester. Benefit-risk analysis suggests influenza immunization may be warranted at any stage of pregnancy during certain pandemics and annually among women with select comorbidities. TIV may also be warranted to protect women against influenza-related hospitalization during the 2nd half of normal pregnancy. Evidence is otherwise insufficient to recommend routine TIV as the standard of practice for all healthy women beginning in early pregnancy.

Treatment of community-acquired lower respiratory tract infections during pregnancy

The incidence of lower respiratory tract infection (LRTI) in women of child-bearing age is approximately 64 per 1000 population. The spectrum of illness ranges from acute bronchitis, which is very common, through influenza virus infection and exacerbations of underlying lung disease, to pneumonia, which, fortunately is uncommon (<1.5% LRTI), but can be severe.

Acute bronchitis is generally mild, self-limiting and usually does not require antibacterial therapy. Influenza virus infection in pregnant women has been recently related to increased hospitalization for acute cardiorespiratory conditions. At present, the safety of the newer neuraminidase inhibitors for the treatment of influenza virus infection has not been established in pregnancy and they are not routinely recommended. In influenza virus infection complicated by pneumonia, antibacterial agents active against Staphylococcus aureus and Streptococcus pneumoniae superinfection should be used.

There are few data on infective complications of asthma or COPD in pregnancy. The latter is rare, as patients with COPD are usually male and aged over 45 years. Management is the same as for nonpregnant patients.

The incidence and mortality of pneumonia in pregnancy is similar to that in nonpregnant patients. Infants born to pregnant patients with pneumonia have been found to be born earlier and weigh less than controls. Risk factors for the development of pneumonia include anemia, asthma and use of antepartum corticosteroids and tocolytic agents. Based on the few available studies, the main pathogens causing pneumonia are S. pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae and viruses. β-Lactam and macrolide antibiotics therefore remain the antibiotics of choice in terms of both pathogen coverage and safety in pregnancy. In HIV-infected pregnant patients, recurrent bacterial pneumonia, but not Pneumocystis carinii pneumonia (PCP), is more common than in nonpregnant patients. Trimethoprim/sulfamethoxazole (cotrimoxazole) has not definitely been associated with adverse clinical outcomes despite theoretical risks. Currently it is still the treatment of choice in PCP, where mortality remains high.

In conclusion, there are few data specifically related to pregnant women with different types of LRTI. Where data are available, no significant differences compared with nonpregnant patients have been identified. In considering the use of any therapeutic agent or investigation in pregnant patients with LRTI, safety aspects must be carefully weighed against potential benefit. Otherwise, management strategies should not differ from those for nonpregnant patients. Further research in this area is warranted.

Influenza epidemics and anencephaly

To explore the postulated association between maternal influenza and congenital defects of the central nervous system, 14 virologically verified epidemics in Finland, 1969-82, were studied. Mothers of 248 anencephalic children were grouped into those whose first trimester had occurred during an epidemic period and those whose pregnancy had commenced during a non-epidemic period. No significant differences in prevalence of anencephaly were noted in these groups.

Role of maternal immunity in the protection of newborn ferrets against infection with a virulent influenza virus

Intranasal infection of newborn ferrets with a virulent strain of influenza virus invariably resulted in their deaths following virus replication to high titre in both lung and nasal turbinates (Collie et al., 1980). However, a similar challenge of newborn ferrets born to mothers immunized by infection with virulent or attenuated viruses resulted in complete protection; no virus replicated in their lungs and little or no virus was isolated from their nasal turbinates. Protection appeared to be antibody-mediated since it was sub-type-specific and milk-derived since newborn ferrets born to non-immune mothers but fostered onto immune mothers exhibited a similar level of protection to neonates born to and suckled by immune mothers.

The effects of maternal influenzal viraemia in late gestation on the conceptus of the pregnant ferret

Pregnant ferrets were inoculated intra-cardially on day 30 of gestation with influenza virus. The animals were sacrificed on days 5 to 11 after inoculation and the products of conception including the uterus were examined virologically and histopathologically. The results indicate that the initial site of infection of the conceptus is the haemophagous organ and that spread occurs from this site to the endometrium, placental labyrinth and fetus. Lesions in the fetus are confined to the liver and respiratory tract. In the liver they may represent either a viral hepatitis or a secondary response to placental damage resulting in the stimulation of erythropoiesis. In the respiratory tract they first occur in the nasal sinuses and upper airways suggesting that infection is via the amniotic fluid rather than via the blood stream. The relevance of these findings to human pregnancy is discussed.

Association of foetal wastage with influenza infection during ferret pregnancy

Inoculation of influenza virus into pregnant ferrets during the late gestational period was investigated. Foetal resorption followed intracardial inoculation of a large dose of influenza virus (10(9.4) EBID50) and a 100-fold lower dose caused lower litter sizes at birth. The possible role of fever in foetal resorptions was largely discounted by 2 observations: a non-pyrexic dose inoculated intracardially into the pregnant ferret still had detrimental effects on foetal viability; influenza virus inoculated intranasally caused a pyrexia but did not affect the progeny. The potential of the ferret as a model for studying the possible adverse effects of influenza during pregnancy is discussed.

Influenza infections during pregnancy in the mouse

The effect of a sublethal dose (0-5 LD50) of influenza A/WSN (H0N1) administered intranasally in the 1st and 3rd weeks of pregnancy was studied in C3H inbred mice. Maternal and neonatal mortality rates were significantly increased by infections in the 3rd week of pregnancy, but not in the 1st week. Infections during the last part of the 1st week significantly depressed the growth rate of neonates. No evidence of viraemia, transplacental transmission or congenital malformations were observed from infections during either week. The results are discussed in terms of a possible model for human influenza infections during pregnancy to determine the potential risks to the mother, fetus and neonate.

The potential advantages and requirements of live attenuated influenza virus vaccines

Live attenuated influenza A virus vaccines are potentially the most efficient and effective method of immunization against epidemic influenza, and offer the only feasible means of mass vaccination at a socially acceptable cost. The advantage of live virus vaccines are described and compared with killed virus vaccines in terms of immune responses, protection and commercial production. The most frequently considered methods of attenuation and their individual drawbacks are discussed with speculation on the future development and rationale of master vaccine strains. Recommended minimal requirements of master vaccine strains and candidate live vaccines are presented as a basis for their eventual evaluation by licensing authorities.

The pregnant ferret as a model for studying the congenital effects of influenza virus infection in utero: infection of foetal tissues in organ culture and in vivo

Organ cultures of ferret foetal tissues showed a similar pattern of susceptibility to influenza virus to that already observed for human foetal tissues (Rosztoczy et al., 1975); respiratory, alimentary and urogenital tissues supported the replication of influenza virus but nervous and lymphopoietic tissues (those which, in man, are associated with foetal or postnatal abnormalities) were insusceptible. In contrast to corresponding human tissues, ferret foetal placenta and amnion readily supported viral replication although both human and ferret umbilical cord were susceptible. In limited experiments, neither the membranes nor the susceptible foetal tissues became infected after intranasal inoculation of pregnant ferrets of various gestational ages. However, after intracardial inoculation of pregnant ferrets with high titre virus (ca 10(9) EBID50) virus was isolated from both foetal membranes and foetuses. The membranes became infected at early, middle and late gestation, but virus appeared to cross the placental barrier to infect foetal tissues only in late gestation. At this stage virus could be isolated not only from those foetal tissues (respiratory, alimentary and urogenital) susceptible in organ culture, but also in small amounts from tissues which were insusceptible in organ culture (heart, lymphopoietic and nervous tissue). Virus was also isolated from foetal membranes and foetuses of late gestation ferrets following intracardial inoculation with a one hundred-fold lower dose of virus which, unlike the higher dose, did not induce a maternal febrile response. The pregnant ferret appears to be a suitable model for investigating the effects on development of foetal infection with influenza virus but it may have disadvantages with regard to the nature and strength of the placental barrier.

The effect of cigarette smoking on susceptibility to epidemic influenza and on serological responses to live attenuated and killed subunit influenza vaccines

The effects of cigarette smoking on the incidence of epidemic influenza and on the serological response to influenza vaccination with killed subunit and live attenuated vaccines have been investigated during comparative vaccine trials in Western Australia. It was found that cigarette smokers with no pre-epidemic haemagglutination-inhibiting (HI) antibody (titres of less than or equal to 12) were significantly more susceptible to epidemic influenza than non-smokers. Smokers were no more susceptible however, if they had possessed detectable pre-epidemic HI antibody. A significantly higher proportion of smokers sero-converted after receiving the live virus vaccine than their non-smoking counterparts, but this could not be correlated with pre-vaccination HI antibody titres. The longevity of the immune response to the subunit vaccine was severely depressed 50 weeks post-vaccination in smokers who had possessed little or no immunity before vaccination (titres of less than or equal to 12). This antiboyd deficit was not observed in live virus vaccines or subunit vaccinees with pre-vaccination HI antibody (titres of greater than or equal to 24). Post-vaccinal symptoms were similar regardless of vaccine group or smoking history.

Comparative trials of live attenuated and detergent split influenza virus vaccines

Comparative clinical trials of live attenuated and detergent-split subunit influenza virus vaccines were undertaken with 1048 volunteers in Western Australia. Volunteers were divided into three main groups, each of which received either live virus vaccine or a saline control administered intranasally, or subunit vaccine injected subcutaneously. No differences were recorded between the three groups in their post-vaccination symptoms. Serum samples were collected at various times up to 50 weeks after vaccination, and antibody titres were measured by haemagglutination-inhibition (HI) tests and, for 231 volunteers, by virus neutralization tests. The two vaccines were almost equivalent in inducing seroconversion in vaccinees with pre-trial HI titres of 96 or less, but the subunit vaccine stimulated a higher geometric mean HI antibody titre. The longevity of the HI antibody response was greater for the live virus vaccine. The height of the response and the longevity of neutralizing antibody were the same for both vaccines. Both vaccines provided a high degree of protection against epidemic A/England/42/72 influenza, and some protection against A/Port Chalmers/1/73 influenza.

Replication of influenza virus in organ cultures of human and simian urogenital tissues and human foetal tissues

A survey of human adult tissues in organ cultures showed that influenza viruses (A/Moscow/1019/65 (h2n2) or a recombinant virus virulent for man (PR/8-A/England/939/69 Clone 7a(H3N2)) could infect uterus, bladder and conjunctiva but not oesophagus under the conditions employed; simian bladder and uterus were also susceptible. These results were similar to those already described for corresponding ferret tissues. Organ cultures of human foetal nasal mucosa, trachea, oesophagus, small and large intestine, and bladder consistently supported replication over 4 days or more with high virus yields. Lung, conjunctiva and umbilical cord were less consistently susceptible and gave lower yields. Placenta and kidney cultures allowed replication of virus in one of 8 and one of 4 experiments respectively, the yields being low and of short duration. Organ cultures of neural tissue (meninges and brain), lymphopoietic tissue (spleen, liver and thymus) and amnion did not support significant viral replication. The results are discussed in relation to possible infection of the foetus in utero with influenza virus.